Don't clear cfp, it causes problems.

thread.c

@@ -599,11 +599,8 @@ thread_cleanup_func_before_exec(void *th_ptr)
     // The thread stack doesn't exist in the forked process:
     th->ec->machine.stack_start = th->ec->machine.stack_end = NULL;
 
-    // Appears to be causing a lot of issues.
-    if (0) {
-      // The vm_stack is `alloca`ed on the thread stack, so it's gone too:
-      rb_ec_clear_vm_stack(th->ec);
-    }
+    // The vm_stack is `alloca`ed on the thread stack, so it's gone too:
+    rb_ec_clear_vm_stack(th->ec);
 }
 
 static void

vm.c

@@ -2714,7 +2714,7 @@ rb_ec_clear_vm_stack(rb_execution_context_t *ec)
     rb_ec_set_vm_stack(ec, NULL, 0);
 
     // Avoid dangling pointers:
-    ec->cfp = NULL;
+    // ec->cfp = NULL;
 }
 
 static void