MXS-852: Fix execution of COM_STMT_FETCH

The COM_STMT_FETCH queries are always executed when the result has not
been fully read. This means that the statement queuing code needs to allow
COM_STMT_FETCH commands to pass if a statement is being executed but the
command queue is empty.

server/modules/protocol/MySQL/mysql_common.c

@@ -1654,5 +1654,6 @@ bool mxs_mysql_command_will_respond(uint8_t cmd)
 {
     return cmd != MYSQL_COM_STMT_SEND_LONG_DATA &&
            cmd != MYSQL_COM_QUIT &&
-           cmd != MYSQL_COM_STMT_CLOSE;
+           cmd != MYSQL_COM_STMT_CLOSE &&
+           cmd != MYSQL_COM_STMT_FETCH;
 }