scripts/get_maintainer.pl: fix possible infinite loop

If MAINTAINERS section entries are misformatted, it was possible to have
an infinite loop.

Correct the defect by always moving the index to the end of section + 1

Also, exit check for exclude as soon as possible.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>