avcodec/pngdec: avoid erroring with sBIT on indexed-color images

Indexed color images use three colors for sBIT, but the function
ff_png_get_nb_channels returns 1 in this case. We should avoid erroring
out on valid files in this scenario.

Regression since 84b454935f.

Signed-off-by: Leo Izen <leo.izen@gmail.com>
Reported-by: Ramiro Polla <ramiro.polla@gmail.com>
Reviewed-by: Marton Balint <cus@passwd.hu>

libavcodec/pngdec.c

@@ -1023,7 +1023,7 @@ static int decode_sbit_chunk(AVCodecContext *avctx, PNGDecContext *s,
         return AVERROR_INVALIDDATA;
     }
 
-    channels = ff_png_get_nb_channels(s->color_type);
+    channels = s->color_type & PNG_COLOR_MASK_PALETTE ? 3 : ff_png_get_nb_channels(s->color_type);
 
     if (bytestream2_get_bytes_left(gb) != channels)
         return AVERROR_INVALIDDATA;