Migration+TLS: Fix crash due to double cleanup

During a TLS connect we see:
  migration_channel_connect calls
  migration_tls_channel_connect
  (calls after TLS setup)
  migration_channel_connect

My previous error handling fix made migration_channel_connect
call migrate_fd_connect in all cases; unfortunately the above
means it gets called twice and crashes doing double cleanup.

Fixes: 688a3dcb

Reported-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180430185943.35714-1-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>