msrledec: use signed pixel_ptr in msrle_decode_pal4

This fixes segmentation faults, when pic->linesize[0] is negative.
In that case 'line * pic->linesize[0] + pixel_ptr' is treated as
unsigned and wraps around.

This reverts commit 7d78a964.
The problem was introduced in commit f7e1367f, which should obsolete
that commit.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
