update decode_fast.c to assert that non-const messages are not frozen

PiperOrigin-RevId: 627393634

upb/wire/internal/decode_fast.c

@@ -231,6 +231,7 @@ UPB_FORCEINLINE
 void* fastdecode_getfield(upb_Decoder* d, const char* ptr, upb_Message* msg,
                           uint64_t* data, uint64_t* hasbits,
                           fastdecode_arr* farr, int valbytes, upb_card card) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
   switch (card) {
     case CARD_s: {
       uint8_t hasbit_index = *data >> 24;
@@ -606,6 +607,7 @@ UPB_NOINLINE
 static const char* fastdecode_verifyutf8(upb_Decoder* d, const char* ptr,
                                          upb_Message* msg, intptr_t table,
                                          uint64_t hasbits, uint64_t data) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
   upb_StringView* dst = (upb_StringView*)data;
   if (!_upb_Decoder_VerifyUtf8Inline(dst->data, dst->size)) {
     _upb_FastDecoder_ErrorJmp(d, kUpb_DecodeStatus_BadUtf8);
@@ -651,6 +653,7 @@ UPB_NOINLINE
 static const char* fastdecode_longstring_noutf8(
     struct upb_Decoder* d, const char* ptr, upb_Message* msg, intptr_t table,
     uint64_t hasbits, uint64_t data) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
   upb_StringView* dst = (upb_StringView*)data;
   FASTDECODE_LONGSTRING(d, ptr, msg, table, hasbits, dst, false);
 }