vcl: fix error state switch for vcl_handle_mq_event

vcl: fix error state switch for vcl_handle_mq_event

When a listen session receives an ACCEPTED message, but then
receives either a RESET or DISCONNECTED message from VPP before the
session is accepted, the listen session state is switched to
VPP_CLOSING or DISCONNECT.
The subsequent CLEANUP message handler attempts to send a
disconneted or reset reply message to VPP, but since the vpp_evt_q
for the listen session is null, this leads to a crash.

Type: fix
Change-Id: Ic51f78f631fe8d15bf8c56b795f4a900c3e2f724
Signed-off-by: wanghanlin <wanghanlin@corp.netease.com>