Avoid a use after free in VM assertion

If the thread for the current EC has been killed, don't check
the VM ptr for the EC (which gets it via the thread), as that will
have already been freed.

Fixes [Bug #16907]

vm_core.h

@@ -1754,6 +1754,7 @@ rb_current_vm(void)
     VM_ASSERT(ruby_current_vm_ptr == NULL ||
 	      ruby_current_execution_context_ptr == NULL ||
 	      rb_ec_thread_ptr(GET_EC()) == NULL ||
+              rb_ec_thread_ptr(GET_EC())->status == THREAD_KILLED ||
 	      rb_ec_vm_ptr(GET_EC()) == ruby_current_vm_ptr);
     return ruby_current_vm_ptr;
 }