RxRPC: Error handling for rxrpc_alloc_connection()

rxrpc_alloc_connection() doesn't return an error code on failure, it just
returns NULL.  IS_ERR(NULL) is false.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>