avcodec/pngdec: use av_mod_uintp2

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/pngdec.c

@@ -787,7 +787,7 @@ static int decode_trns_chunk(AVCodecContext *avctx, PNGDecContext *s,
 
         for (i = 0; i < length / 2; i++) {
             /* only use the least significant bits */
-            v = bytestream2_get_be16(&s->gb) & ((1 << s->bit_depth) - 1);
+            v = av_mod_uintp2(bytestream2_get_be16(&s->gb), s->bit_depth);
 
             if (s->bit_depth > 8)
                 AV_WB16(&s->transparent_color_be[2 * i], v);