scripts/Lindent: handle missing indent gracefully

If indent is not found, bail out immediately instead of spitting random
shell script error messages.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>