Fix crash of gallery on showing details

The exif parser adds a terminating zero for string values. This increases the
byte count for that entry beyond the set byte count. Causing a null value
in e.g. the "Model" field for images taken with e.g. a Samsung Wave 2.

Change-Id: Ifb6ee4d5b4b71e84cb2a5df459d3716543e25efe