avcodec/pngdec: Use ff_set_dimensions()

Fixes OOM
Fixes: 1314/clusterfuzz-testcase-minimized-4621997222920192

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pngdec.c

@@ -622,8 +622,9 @@ static int decode_idat_chunk(AVCodecContext *avctx, PNGDecContext *s,
     }
     if (!(s->pic_state & PNG_IDAT)) {
         /* init image info */
-        avctx->width  = s->width;
-        avctx->height = s->height;
+        ret = ff_set_dimensions(avctx, s->width, s->height);
+        if (ret < 0)
+            return ret;
 
         s->channels       = ff_png_get_nb_channels(s->color_type);
         s->bits_per_pixel = s->bit_depth * s->channels;