vm.c (stack_copy) check if src is NULL before memcpy(); ref #5777

Because 2nd argument of memcpy() is declared to never be null.

src/vm.c

@@ -102,7 +102,9 @@ stack_clear(mrb_value *from, size_t count)
 static inline void
 stack_copy(mrb_value *dst, const mrb_value *src, size_t size)
 {
-  memcpy(dst, src, sizeof(mrb_value)*size);
+  if (src) {
+    memcpy(dst, src, sizeof(mrb_value)*size);
+  }
 }
 
 static void