libmount: make mnt_get_filesystems() more robust [clang analyze]

The mnt_free_filesystems() relies on NULL as the last item in the
filesystems array. It's necessary to keep NULL there after failed
strdup() too, because we call mnt_free_filesystems() to deallocate
after error.

Signed-off-by: Karel Zak <kzak@redhat.com>

libmount/src/utils.c

@@ -550,10 +550,10 @@ static int add_filesystem(char ***filesystems, char *name)
 		*filesystems = x;
 	}
 	name = strdup(name);
-	if (!name)
-		goto err;
 	(*filesystems)[n] = name;
 	(*filesystems)[n + 1] = NULL;
+	if (!name)
+		goto err;
 	return 0;
 err:
 	mnt_free_filesystems(*filesystems);