fixes some (not all) buffer overreads during decoding pentax makernote entries.

This should fix:
https://sourceforge.net/p/libexif/bugs/125/ CVE-2016-6328

Cherrypicked from
https://github.com/libexif/libexif/commit/41bd04234b104312f54d25822f68738b

Bug: 162602132
Test: Android camera tests continue to pass
Change-Id: Ib7d9d3afe67aa37bfa6d23ff88f6334e1f8f4396
(cherry picked from commit 836cfdb99e8aacf0c49a714141d80f66c2ebada0)