tiff: Check that there is no aliasing in pixel format selection

Fixes possible issues with unexpected bpp/bppcount values.

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Bug-Id: CVE-2014-8544
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
