Ensure ruby_xfree won't segfault if called after vm_destruct

[Bug #19580]

The real-world scenario motivating this change is libxml2's pthread
code which uses `pthread_key_create` to set up a destructor that is
called at thread exit to free thread-local storage.

There is a small window of time -- after ruby_vm_destruct but before
the process exits -- in which a pthread may exit and the destructor is
called, leading to a segfault.

Please note that this window of time may be relatively large if
`atexit` is being used.

gc.c

@@ -12581,9 +12581,17 @@ ruby_xrealloc2_body(void *ptr, size_t n, size_t size)
 void
 ruby_sized_xfree(void *x, size_t size)
 {
-    if (x) {
+    if (LIKELY(x)) {
+        /* It's possible for a C extension's pthread destructor function set by pthread_key_create
+         * to be called after ruby_vm_destruct and attempt to free memory. Fall back to mimfree in
+         * that case. */
+        if (LIKELY(GET_VM())) {
             objspace_xfree(&rb_objspace, x, size);
         }
+        else {
+            ruby_mimfree(x);
+        }
+    }
 }
 
 void