In `upb_MiniTableExtension_GetSubMessage`, return NULL for non-messages.

This parallels the logic in `upb_MiniTable_SubMessage`.

PiperOrigin-RevId: 632277936

upb/mini_table/internal/extension.h

@@ -8,6 +8,7 @@
 #ifndef UPB_MINI_TABLE_INTERNAL_EXTENSION_H_
 #define UPB_MINI_TABLE_INTERNAL_EXTENSION_H_
 
+#include <stddef.h>
 #include <stdint.h>
 
 #include "upb/base/descriptor_constants.h"
@@ -41,6 +42,9 @@ upb_MiniTableExtension_Number(const struct upb_MiniTableExtension* e) {
 
 UPB_API_INLINE const struct upb_MiniTable* upb_MiniTableExtension_GetSubMessage(
     const struct upb_MiniTableExtension* e) {
+  if (upb_MiniTableExtension_CType(e) != kUpb_CType_Message) {
+    return NULL;
+  }
   return upb_MiniTableSub_Message(e->UPB_PRIVATE(sub));
 }