lib/btrfs: avoid NULL-dereference

btrfs.c:42:13: warning: use of NULL 'cmd' where non-null expected [CWE-476] [-Wanalyzer-null-argument]

Reviewed-by: Alejandro Colomar <alx@kernel.org>
Cherry-picked-from: 54ab542887
Link: <https://github.com/shadow-maint/shadow/pull/770>
Reviewed-by: Michael Vetter <jubalh@iodoru.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/btrfs.c

@@ -39,7 +39,7 @@ static int run_btrfs_subvolume_cmd(const char *subcmd, const char *arg1, const c
 		NULL
 	};
 
-	if (access(cmd, X_OK)) {
+	if (!cmd || access(cmd, X_OK)) {
 		return 1;
 	}