Fix for CONC-504: reset stmt->result.rows when executing mysql_stmt_next_result

While in text protocol the number of rows is resetted in mysql_store/use_result
in binary protocol we need to explicitly reset it when switching to next result set.

libmariadb/mariadb_stmt.c

@@ -2374,6 +2374,7 @@ int STDCALL mysql_stmt_next_result(MYSQL_STMT *stmt)
   }
 
   stmt->field_count= stmt->mysql->field_count;
+  stmt->result.rows= 0;
 
   return(rc);
 }