upb: fix uninitialized upb_MessageValue buffer bugs

Fixes protocolbuffers#18045

This should also cover googleapis/proto-plus-python#483 once we release it.

PiperOrigin-RevId: 671934556

upb/json/decode.c

@@ -711,7 +711,8 @@ static upb_MessageValue jsondec_int(jsondec* d, const upb_FieldDef* f) {
 
 /* Parse UINT32 or UINT64 value. */
 static upb_MessageValue jsondec_uint(jsondec* d, const upb_FieldDef* f) {
-  upb_MessageValue val = {0};
+  upb_MessageValue val;
+  memset(&val, 0, sizeof(val));
 
   switch (jsondec_peek(d)) {
     case JD_NUMBER: {
@@ -748,7 +749,8 @@ static upb_MessageValue jsondec_uint(jsondec* d, const upb_FieldDef* f) {
 /* Parse DOUBLE or FLOAT value. */
 static upb_MessageValue jsondec_double(jsondec* d, const upb_FieldDef* f) {
   upb_StringView str;
-  upb_MessageValue val = {0};
+  upb_MessageValue val;
+  memset(&val, 0, sizeof(val));
 
   switch (jsondec_peek(d)) {
     case JD_NUMBER:

upb/reflection/message.c

@@ -138,7 +138,8 @@ bool upb_Message_Next(const upb_Message* msg, const upb_MessageDef* m,
   const upb_MiniTable* mt = upb_MessageDef_MiniTable(m);
   size_t i = *iter;
   size_t n = upb_MiniTable_FieldCount(mt);
-  const upb_MessageValue zero = {0};
+  upb_MessageValue zero;
+  memset(&zero, 0, sizeof(zero));
   UPB_UNUSED(ext_pool);
 
   // Iterate over normal fields, returning the first one that is set.