btrfs-progs: remove unnecessary NULL check after get_df()

If get_df() returns 0, "sargs" surely points to malloc'ed region.
So NULL check of sargs is not necessary.

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

cmds-filesystem.c

@@ -238,7 +238,7 @@ static int cmd_df(int argc, char **argv)
 	}
 	ret = get_df(fd, &sargs);
 
-	if (!ret && sargs) {
+	if (ret == 0) {
 		print_df(sargs);
 		free(sargs);
 	} else {