packet.c: fix possible NULL pointer de-reference within listen_state
packet.c: fix possible NULL pointer de-reference within listen_state

Fixes VS2012 code analysis warning C6011:
dereferencing NULL pointer 'listen_state->channel'. libssh2 packet.c 221

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2