fix SEGV touching uninitialized local variable

This imemo_name is used uninitialized because the switch
above does not cover all possible imemo types.

(lldb) run
Process 26068 launched: './miniruby' (x86_64)
Process 26068 stopped
* thread #1: tid = 0x14ba96, 0x00007fff8a402132 libsystem_c.dylib`strlen + 18, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0xfffffffffffffff0)
    frame #0: 0x00007fff8a402132 libsystem_c.dylib`strlen + 18
libsystem_c.dylib`strlen:
->  0x7fff8a402132 <+18>: pcmpeqb (%rdi), %xmm0
    0x7fff8a402136 <+22>: pmovmskb %xmm0, %esi
    0x7fff8a40213a <+26>: andq   $0xf, %rcx
    0x7fff8a40213e <+30>: orq    $-0x1, %rax
(lldb) bt
* thread #1: tid = 0x14ba96, 0x00007fff8a402132 libsystem_c.dylib`strlen + 18, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0xfffffffffffffff0)
  * frame #0: 0x00007fff8a402132 libsystem_c.dylib`strlen + 18
    frame #1: 0x00000001001f1531 miniruby`BSD_vfprintf(fp=0x00007fff5fbfc9e0, fmt0="%s %s", ap=0x00007fff5fbfcbf0) + 5873 at vsnprintf.c:1026
    frame #2: 0x00000001001ef213 miniruby`ruby_do_vsnprintf(str="0x0000000100f46450 [0    ] T_IMEMO", n=256, fmt="%s %s", ap=0x00007fff5fbfcbf0) + 131 at sprintf.c:1285
    frame #3: 0x00000001001ef3ea miniruby`ruby_snprintf(str="0x0000000100f46450 [0    ] T_IMEMO", n=256, fmt="%s %s") + 426 at sprintf.c:1300
    frame #4: 0x00000001000bdc61 miniruby`rb_raw_obj_info(buff="0x0000000100f46450 [0    ] T_IMEMO", buff_size=256, obj=4310983760) + 2353 at gc.c:9376
    frame #5: 0x00000001000b7bff miniruby`obj_info(obj=4310983760) + 95 at gc.c:9428
    frame #6: 0x00000001000c1658 miniruby`newobj_init(klass=0, flags=36890, v1=0, v2=4303040512, v3=4310983800, wb_protected=1, objspace=0x00000001007ee280, obj=4310983760) + 424 at gc.c:1887
    frame #7: 0x00000001000b4469 miniruby`newobj_of(klass=0, flags=36890, v1=0, v2=4303040512, v3=4310983800, wb_protected=1) + 217 at gc.c:1970
    frame #8: 0x00000001000b45eb miniruby`rb_imemo_new(type=imemo_ast, v1=0, v2=4303040512, v3=4310983800, v0=0) + 75 at gc.c:2017
    frame #9: 0x000000010011daed miniruby`rb_ast_new + 61 at node.c:1146
    frame #10: 0x0000000100160e15 miniruby`rb_parser_compile_file_path(vparser=4310984400, fname=4310984960, file=4310984080, start=1) + 53 at parse.y:5776
    frame #11: 0x00000001001e18ea miniruby`load_file_internal(argp_v=140734799795024) + 1834 at ruby.c:1907
    frame #12: 0x00000001000a1bb5 miniruby`rb_ensure(b_proc=(miniruby`load_file_internal at ruby.c:1795), data1=140734799795024, e_proc=(miniruby`restore_load_file at ruby.c:2007), data2=140734799795024) + 245 at eval.c:1037
    frame #13: 0x00000001001df4a4 miniruby`load_file(parser=4310984400, fname=4310984960, f=4310984080, script=1, opt=0x00007fff5fbfda28) + 100 at ruby.c:2026
    frame #14: 0x00000001001e084e miniruby`process_options(argc=0, argv=0x00007fff5fbfdc00, opt=0x00007fff5fbfda28) + 3454 at ruby.c:1682
    frame #15: 0x00000001001dfaae miniruby`ruby_process_options(argc=2, argv=0x00007fff5fbfdbf0) + 238 at ruby.c:2257
    frame #16: 0x000000010009ff43 miniruby`ruby_options(argc=2, argv=0x00007fff5fbfdbf0) + 211 at eval.c:105
    frame #17: 0x0000000100000989 miniruby`main(argc=2, argv=0x00007fff5fbfdbf0) + 105 at main.c:42
    frame #18: 0x00007fff88eda5ad libdyld.dylib`start + 1
(lldb) up 4
frame #4: 0x00000001000bdc61 miniruby`rb_raw_obj_info(buff="0x0000000100f46450 [0    ] T_IMEMO", buff_size=256, obj=4310983760) + 2353 at gc.c:9376
   9373 #undef IMEMO_NAME
   9374               default: UNREACHABLE;
   9375             }
-> 9376             snprintf(buff, buff_size, "%s %s", buff, imemo_name);
   9377
   9378             switch (imemo_type(obj)) {
   9379               case imemo_ment: {
(lldb) p imemo_name
(const char *) $0 = 0xffffffffffffffff
(lldb) p imemo_type(obj)
(imemo_type) $1 = imemo_ast
(lldb)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61566 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -9359,7 +9359,7 @@ rb_raw_obj_info(char *buff, const int buff_size, VALUE obj)
 	    break;
 	  }
 	  case T_IMEMO: {
-	    const char *imemo_name;
+	    const char *imemo_name = "\0";
 	    switch (imemo_type(obj)) {
 #define IMEMO_NAME(x) case imemo_##x: imemo_name = #x; break;
 		IMEMO_NAME(env);