Make rpc_disconnect() a no-op if the session is already disconnected

This addresses a bug causing a segfault if we destroy the nfs context/
disconnect the session while the mount_8_cb callbacks for checking the
filehandle for nested mountpoints are still in flight.

Issue found and reported by doktorstick

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>

lib/socket.c

@@ -583,6 +583,10 @@ int rpc_disconnect(struct rpc_context *rpc, const char *error)
 {
 	assert(rpc->magic == RPC_CONTEXT_MAGIC);
 
+	/* Do not re-disconnect if we are already disconnected */
+	if (!rpc->is_connected) {
+		return 0;
+	}
 	rpc_unset_autoreconnect(rpc);
 
 	if (rpc->fd != -1) {