btrfs-progs: check result of first_cache_extent

If the tree's empty, we'll get NULL and dereference it.

Resolves-Coverity-CID: 1248828
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

cmds-check.c

@@ -8075,6 +8075,8 @@ static void free_roots_info_cache(void)
 		struct root_item_info *rii;
 
 		entry = first_cache_extent(roots_info_cache);
+		if (!entry)
+			break;
 		remove_cache_extent(roots_info_cache, entry);
 		rii = container_of(entry, struct root_item_info, cache_extent);
 		free(rii);