libavcodec/exr : fix channel size calculation for uint32 channel

uint32 need 4 bytes not 1.
Fix decoding when there is half/float and uint32 channel.

This fixes crashes due to pointer corruption caused by invalid writes.

The problem was introduced in commit
03152e74dfdc7f438cb4a10402c4de744e807e22.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
