avcodec/ffv1: Simplify fold()

No speed difference, or slightly faster (the difference is too small so it may be noise
that this appears faster)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ffv1.h

@@ -153,9 +153,7 @@ static av_always_inline int fold(int diff, int bits)
     if (bits == 8)
         diff = (int8_t)diff;
     else {
-        diff +=  1 << (bits  - 1);
-        diff  = av_mod_uintp2(diff, bits);
-        diff -=  1 << (bits  - 1);
+        diff = sign_extend(diff, bits);
     }
 
     return diff;