avcodec/x86/h264_weight: handle weight1=128

Fix ticket4596

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit e100966575)

libavcodec/x86/h264_weight.asm

@@ -135,8 +135,11 @@ WEIGHT_FUNC_HALF_MM 8, 8
     add  off_regd, 1
     or   off_regd, 1
     add        r4, 1
+    cmp        r6d, 128
+     je .nonnormal
     cmp        r5, 128
      jne .normal
+.nonnormal
     sar        r5, 1
     sar        r6, 1
     sar  off_regd, 1