avoid buffer overflow in vm_check_canary

ec->cfp->iseq might not exist at the very beginning of a thread.

=================================================================
==82954==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7fc86f334810 at pc 0x55ceaf013125 bp 0x7ffe2eddbbf0 sp 0x7ffe2eddbbe8
READ of size 8 at 0x7fc86f334810 thread T0
    #0 0x55ceaf013124 in vm_check_canary vm_insnhelper.c:217:24
    #1 0x55ceaefb4796 in vm_push_frame vm_insnhelper.c:276:5
    #2 0x55ceaf0124bd in th_init vm.c:2661:5
    #3 0x55ceaf00d5eb in ruby_thread_init vm.c:2690:5
    #4 0x55ceaf00d4b1 in rb_thread_alloc vm.c:2703:5
    #5 0x55ceaef0038b in thread_s_new thread.c:872:20
    #6 0x55ceaf04d8c1 in call_cfunc_m1 vm_insnhelper.c:2041:12
    #7 0x55ceaf03118d in vm_call_cfunc_with_frame vm_insnhelper.c:2207:11
    #8 0x55ceaf017985 in vm_call_cfunc vm_insnhelper.c:2225:12
    #9 0x55ceaf01548b in vm_call_method_each_type vm_insnhelper.c:2560:9
    #10 0x55ceaf014c96 in vm_call_method vm_insnhelper.c:2686:13
    #11 0x55ceaefb5de4 in vm_call_general vm_insnhelper.c:2730:12
    #12 0x55ceaf03c868 in vm_sendish vm_insnhelper.c:3623:11
    #13 0x55ceaefc95bb in vm_exec_core insns.def:771:11
    #14 0x55ceaf006700 in rb_vm_exec vm.c:1892:22
    #15 0x55ceaf00acbf in rb_iseq_eval_main vm.c:2151:11
    #16 0x55ceaea250ca in ruby_exec_internal eval.c:262:2
    #17 0x55ceaea2498b in ruby_exec_node eval.c:326:12
    #18 0x55ceaea247d0 in ruby_run_node eval.c:318:25
    #19 0x55ceae88c486 in main main.c:42:9
    #20 0x7fc874330b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #21 0x55ceae7e5289 in _start (miniruby+0x15f289)

0x7fc86f334810 is located 16 bytes to the right of 1048576-byte region [0x7fc86f234800,0x7fc86f334800)
allocated by thread T0 here:
    #0 0x55ceae85d56d in malloc (miniruby+0x1d756d)
    #1 0x55ceaea71d12 in objspace_xmalloc0 gc.c:9416:5
    #2 0x55ceaea71cd2 in ruby_xmalloc2_body gc.c:9623:12
    #3 0x55ceaea7d09c in ruby_xmalloc2 gc.c:11479:12
    #4 0x55ceaf00c3b7 in rb_thread_recycle_stack vm.c:2462:12
    #5 0x55ceaf012256 in th_init vm.c:2656:29
    #6 0x55ceaf00d5eb in ruby_thread_init vm.c:2690:5
    #7 0x55ceaf00d4b1 in rb_thread_alloc vm.c:2703:5
    #8 0x55ceaef0038b in thread_s_new thread.c:872:20
    #9 0x55ceaf04d8c1 in call_cfunc_m1 vm_insnhelper.c:2041:12
    #10 0x55ceaf03118d in vm_call_cfunc_with_frame vm_insnhelper.c:2207:11
    #11 0x55ceaf017985 in vm_call_cfunc vm_insnhelper.c:2225:12
    #12 0x55ceaf01548b in vm_call_method_each_type vm_insnhelper.c:2560:9
    #13 0x55ceaf014c96 in vm_call_method vm_insnhelper.c:2686:13
    #14 0x55ceaefb5de4 in vm_call_general vm_insnhelper.c:2730:12
    #15 0x55ceaf03c868 in vm_sendish vm_insnhelper.c:3623:11
    #16 0x55ceaefc95bb in vm_exec_core insns.def:771:11
    #17 0x55ceaf006700 in rb_vm_exec vm.c:1892:22
    #18 0x55ceaf00acbf in rb_iseq_eval_main vm.c:2151:11
    #19 0x55ceaea250ca in ruby_exec_internal eval.c:262:2
    #20 0x55ceaea2498b in ruby_exec_node eval.c:326:12
    #21 0x55ceaea247d0 in ruby_run_node eval.c:318:25
    #22 0x55ceae88c486 in main main.c:42:9
    #23 0x7fc874330b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310

SUMMARY: AddressSanitizer: heap-buffer-overflow vm_insnhelper.c:217:24 in vm_check_canary
Shadow bytes around the buggy address:
  0x0ff98de5e8b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ff98de5e8c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ff98de5e8d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ff98de5e8e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ff98de5e8f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0ff98de5e900: fa fa[fa]fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff98de5e910: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff98de5e920: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff98de5e930: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff98de5e940: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff98de5e950: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==82954==ABORTING

vm_insnhelper.c

@@ -214,6 +214,10 @@ vm_check_canary(const rb_execution_context_t *ec, VALUE *sp)
     if (! LIKELY(vm_stack_canary_was_born)) {
         return; /* :FIXME: isn't it rather fatal to enter this branch?  */
     }
+    else if ((VALUE *)reg_cfp == ec->vm_stack + ec->vm_stack_size) {
+        /* This is at the very beginning of a thread. cfp does not exist. */
+        return;
+    }
     else if (! (iseq = GET_ISEQ())) {
         return;
     }