ipc-client: free payload after sending it over the socket

Fixes memory leaks in the form of:

Direct leak of 20 byte(s) in 1 object(s) allocated from:
    #0 0x7f5f7c2f4f30 in __interceptor_malloc (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xedf30)
    #1 0x563c7995b36a in join_args ../common/stringop.c:268
    #2 0x563c798a6a1a in main ../sway/main.c:348
    #3 0x7f5f7b4d609a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2409a)

sway/main.c

@@ -347,6 +347,7 @@ int main(int argc, char **argv) {
 		}
 		char *command = join_args(argv + optind, argc - optind);
 		run_as_ipc_client(command, socket_path);
+		free(command);
 		return 0;
 	}