checkpatch: indent checks -- stop when we run out of continuation lines

Ensure we terminate when there are no futher continuation lines when
trying to determine relative indent of conditionals and their blocks.

Reported-by: John Daiker <daikerjohn@gmail.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>