pgssubdec: Check RLE size before copying

Make sure the buffer size does not exceed the expected
RLE size.

Prevent an out of array bound write.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Bug-Id: CVE-2013-0852

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit a1f7844a11010d8552c75424d1a831b37a0ae5d9)
Signed-off-by: Diego Biurrun <diego@biurrun.de>