ssh: free the session on init failures

Ensure to clear the session object in case the libssh2 initialization
fails.

It could be argued that the libssh2 error function should be called to
get a proper error message in this case. But since the only error path
in libssh2_knownhost_init() is memory a allocation failure it's safest
to avoid since the libssh2 error handling allocates memory.

Closes #3179
Reviewed-by: Daniel Stenberg <daniel@haxx.se>