Fix out of boundary access with exif (#617)

* Fix out of boundary access

The while-loop is not finished when pos is set to EXIF_MAX_DATA.
Instead, the loop continues and therefore tries to access data outside
of the array.

This is triggered when compiled with exif=1 and asan:

$ feh --draw-exif image.jpg

* Fixed formatting

No functional change but makes previous commit easier verifiable
(independent of tab space setup).

* Call break; instead of setting pos2 to a magic value

This is in line with the following else clause

* Another cosmetic adjustment

Co-authored-by: Daniel Friesel <derf@finalrewind.org>