signed integer overflow is undefined in C

Fixes the following gcc warnings:
In file included from libavcodec/hevcpred.c:27:0:
libavcodec/hevcpred_template.c: In function ‘intra_pred_8’:
libavcodec/hevcpred_template.c:302:9: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
In file included from libavcodec/hevcpred.c:31:0:
libavcodec/hevcpred_template.c: In function ‘intra_pred_9’:
libavcodec/hevcpred_template.c:302:9: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
In file included from libavcodec/hevcpred.c:35:0:
libavcodec/hevcpred_template.c: In function ‘intra_pred_10’:
libavcodec/hevcpred_template.c:302:9: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>