socket: Fixed use-after-free.
socket: Fixed use-after-free.

When s->callbacks->exception() was called in ssh_socket_pollcallback()
we had a use after free bug.
(cherry picked from commit 986676378943353cdcf156493812737dc91befdd)

mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh