upb: fix the map encoder to stop dying under ubsan

PiperOrigin-RevId: 601004095

upb/message/map_sorter.c

@@ -113,6 +113,7 @@ static bool _upb_mapsorter_resize(_upb_mapsorter* s, _upb_sortedmap* sorted,
 bool _upb_mapsorter_pushmap(_upb_mapsorter* s, upb_FieldType key_type,
                             const upb_Map* map, _upb_sortedmap* sorted) {
   int map_size = _upb_Map_Size(map);
+  UPB_ASSERT(map_size);
 
   if (!_upb_mapsorter_resize(s, sorted, map_size)) return false;
 

upb/text/encode.c

@@ -357,6 +357,8 @@ static void txtenc_map(txtenc* e, const upb_Map* map, const upb_FieldDef* f) {
       txtenc_mapentry(e, key, val, f);
     }
   } else {
+    if (upb_Map_Size(map) == 0) return;
+
     const upb_MessageDef* entry = upb_FieldDef_MessageSubDef(f);
     const upb_FieldDef* key_f = upb_MessageDef_Field(entry, 0);
     _upb_sortedmap sorted;

upb/wire/encode.c

@@ -439,7 +439,7 @@ static void encode_map(upb_encstate* e, const upb_Message* msg,
       upb_MiniTableSub_Message(subs[f->UPB_PRIVATE(submsg_index)]);
   UPB_ASSERT(layout->UPB_PRIVATE(field_count) == 2);
 
-  if (map == NULL) return;
+  if (!map || !upb_Map_Size(map)) return;
 
   if (e->options & kUpb_EncodeOption_Deterministic) {
     _upb_sortedmap sorted;