restore upb_MapIterator_Done()

It turns out that we still need this for PHP. Oops.

PiperOrigin-RevId: 490034622

upb/collections/map.c

@@ -92,6 +92,14 @@ bool upb_MapIterator_Next(const upb_Map* map, size_t* iter) {
   return _upb_map_next(map, iter);
 }
 
+bool upb_MapIterator_Done(const upb_Map* map, size_t iter) {
+  upb_strtable_iter i;
+  UPB_ASSERT(iter != kUpb_Map_Begin);
+  i.t = &map->table;
+  i.index = iter;
+  return upb_strtable_done(&i);
+}
+
 // Returns the key and value for this entry of the map.
 upb_MessageValue upb_MapIterator_Key(const upb_Map* map, size_t iter) {
   upb_strtable_iter i;

upb/collections/map.h

@@ -107,6 +107,11 @@ bool upb_Map_Next(const upb_Map* map, upb_MessageValue* key,
 // Advances to the next entry. Returns false if no more entries are present.
 bool upb_MapIterator_Next(const upb_Map* map, size_t* iter);
 
+/* Returns true if the iterator still points to a valid entry, or false if the
+ * iterator is past the last element. It is an error to call this function with
+ * kUpb_Map_Begin (you must call next() at least once first). */
+bool upb_MapIterator_Done(const upb_Map* map, size_t iter);
+
 /* Returns the key and value for this entry of the map. */
 upb_MessageValue upb_MapIterator_Key(const upb_Map* map, size_t iter);
 upb_MessageValue upb_MapIterator_Value(const upb_Map* map, size_t iter);