btrfs-progs: let btrfs_free_path accept NULL

Same in kernel and matches semantics of free().

Resolves-Coverity-CID: 1054881
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

ctree.c

@@ -48,6 +48,8 @@ struct btrfs_path *btrfs_alloc_path(void)
 
 void btrfs_free_path(struct btrfs_path *p)
 {
+	if (!p)
+		return;
 	btrfs_release_path(p);
 	kfree(p);
 }