Better estimate memsize of RE2::MatchData

As `matches` is only a pointer to the first StringPiece match, ensure we
multiply it by the number of matches to get a better estimate of the
size of the `RE2::MatchData` object.

ext/re2/re2.cc

@@ -156,7 +156,7 @@ static size_t re2_matchdata_memsize(const void *data) {
   const re2_matchdata *self = (const re2_matchdata *)data;
   size_t size = sizeof(re2_matchdata);
   if (self->matches) {
-    size += sizeof(self->matches);
+    size += sizeof(self->matches) * self->number_of_matches;
   }
 
   return size;