exif_process_APP12: Fix length calculation for second tag to prevent information leak

Summary:
buffer + 2+l1+1 points to the second tag. The maximum length of the buffer is length. Hence, the max length of the tag is length - (2+l1+1). Instead, the length passed is length - 2 - l1 + 1 which leads to a 2 byte overflow. If the last character of buffer is not null, this would lead to an out of bounds read of size 2.

Fixes CVE-2019-11925

Reviewed By: fredemmott

Differential Revision: D16930333

fbshipit-source-id: aad9cccaef3c678abc53bfd549aff6b582082a90