avcodec/utils: Fix encoder allocation size

Fixes Ticket2645
Found-by: Darrell Walisser
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 175e916fa20b7887bdb29809817985e481ae0888)

Conflicts:
	libavcodec/utils.c
alacenc: Fix missing sign_extend()

Fixes ticket #2497

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 8aea2f05dc56f7e7d60767dd27ba8e846a05e8ae)