avcodec/lossless_videodsp: fix output of add_hfyu_left_pred_int16_c()

It is now bitexact with the ssse3 and sse4.1 versions of the function.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/lossless_videodsp.c

@@ -100,15 +100,15 @@ static int add_hfyu_left_pred_int16_c(uint16_t *dst, const uint16_t *src, unsign
 
     for(i=0; i<w-1; i++){
         acc+= src[i];
-        dst[i]= acc & mask;
+        dst[i]= acc &= mask;
         i++;
         acc+= src[i];
-        dst[i]= acc & mask;
+        dst[i]= acc &= mask;
     }
 
     for(; i<w; i++){
         acc+= src[i];
-        dst[i]= acc & mask;
+        dst[i]= acc &= mask;
     }
 
     return acc;