dsputil/pngdsp: fix signed/unsigned type in end comparison

Fixes out of array accesses and integer overflows.

(cherry picked from commit d1916d13e28b87f4b1b214231149e12e1d536b4b)
Adresses: CVE-2013-7010, CVE-2013-7014

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
flashsv: Check diff_start diff_height values

Fix out of array accesses.

Found-by: ami_stuff
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Adresses: CVE-2013-7015
(cherry picked from commit 57070b1468edc6ac8cb3696c817f3c943975d4c1)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>