test_bug11909 case from mysql_client_test.c lead to crash

bt：
(gdb) bt
#0 0x00007f06b17ee6ab in raise () from /lib64/libpthread.so.0
#1 0x00007f06ad2ddc42 in RWBackend::write (this=0x7f066c024360, buffer=0x7f066c0141b0, type=maxscale::Backend::EXPECT_RESPONSE)
at /home/xiangzhong.wxd/gitlab/MaxScale/server/modules/routing/readwritesplit/rwsplitsession.cc:91
#2 0x00007f06ad2e4135 in handle_got_target (inst=0x1ba0740, rses=0x7f066c0232c0, querybuf=0x7f066c024b10, target=std::tr1::shared_ptr (count 4) 0x7f066c024360, store=false)
at /home/xiangzhong.wxd/gitlab/MaxScale/server/modules/routing/readwritesplit/rwsplit_route_stmt.cc:1179
#3 0x00007f06ad2e0bbd in route_single_stmt (inst=0x1ba0740, rses=0x7f066c0232c0, querybuf=0x7f066c024b10, info=...)
at /home/xiangzhong.wxd/gitlab/MaxScale/server/modules/routing/readwritesplit/rwsplit_route_stmt.cc:270
#4 0x00007f06ad2d79b9 in routeQuery (instance=0x1ba0740, router_session=0x7f066c0232c0, querybuf=0x7f066c024b10)
at /home/xiangzhong.wxd/gitlab/MaxScale/server/modules/routing/readwritesplit/readwritesplit.cc:964
#5 0x00007f06aba196ba in route_by_statement (session=0x7f066c012580, capabilities=4294967431, p_readbuf=0x7f067fffb560)
at /home/xiangzhong.wxd/gitlab/MaxScale/server/modules/protocol/MySQL/mariadbclient/mysql_client.cc:1564
#6 0x00007f06aba17d84 in gw_read_finish_processing (dcb=0x7f066c013e90, read_buffer=0x0, capabilities=4294967431)
at /home/xiangzhong.wxd/gitlab/MaxScale/server/modules/protocol/MySQL/mariadbclient/mysql_client.cc:1063
#7 0x00007f06aba17b25 in gw_read_normal_data (dcb=0x7f066c013e90, read_buffer=0x7f066c0141b0, nbytes_read=13)
at /home/xiangzhong.wxd/gitlab/MaxScale/server/modules/protocol/MySQL/mariadbclient/mysql_client.cc:1005

I think this may be a problem, although it only appears in debug patterns.