vhost: fix use after free

A "return" is missing on error, which could lead to a "use after free"
issue (about var "conn").

Coverity issue: 143476
Fixes: 65388b43f5 ("vhost: fix fd leaks for vhost-user server mode")

Reported-by: John McNamara <john.mcnamara@intel.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

lib/librte_vhost/socket.c

@@ -242,6 +242,7 @@ vhost_user_add_connection(int fd, struct vhost_user_socket *vsocket)
 		RTE_LOG(ERR, VHOST_CONFIG,
 			"failed to add fd %d into vhost server fdset\n",
 			fd);
+		return;
 	}
 
 	pthread_mutex_lock(&vsocket->conn_mutex);