vm.c: add runtime type check for OP_STRCAT

Replace mrb_assert with mrb_ensure_string_type to catch codegen bugs
even in release builds. This prevents null-dereference crashes when
OP_STRCAT receives a non-string first operand due to compiler bugs.

Consistent with OP_HASH which uses mrb_ensure_hash_type for similar
type safety.

Co-authored-by: Claude <noreply@anthropic.com>

src/vm.c

@@ -3123,7 +3123,7 @@ RETRY_TRY_BLOCK:
     }
 
     CASE(OP_STRCAT, B) {
-      mrb_assert(mrb_string_p(regs[a]));
+      mrb_ensure_string_type(mrb, regs[a]);
       mrb_str_concat(mrb, regs[a], regs[a+1]);
       ci = mrb->c->ci;
       NEXT;