Fix a valgrind warning. For some reason it never popped up before.

sql/sql_class.cc:
  Fix a valgrind warning (row_count_func is used before it was initialized
  in SQLCOM_CALL)

sql/sql_class.cc

@@ -400,6 +400,7 @@ THD::THD()
   used_tables=0;
   cuted_fields= sent_row_count= row_count= 0L;
   limit_found_rows= 0;
+  row_count_func= -1;
   statement_id_counter= 0UL;
 #ifdef ERROR_INJECT_SUPPORT
   error_inject_value= 0UL;