avformat/apng: fix setting frame delay when max_fps is set to no limit

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 874eb012)

libavformat/apngdec.c

@@ -269,7 +269,7 @@ static int decode_fctl_chunk(AVFormatContext *s, APNGDemuxContext *ctx, AVPacket
     /* default is hundredths of seconds */
     if (!delay_den)
         delay_den = 100;
-    if (!delay_num || delay_den / delay_num > ctx->max_fps) {
+    if (!delay_num || (ctx->max_fps && delay_den / delay_num > ctx->max_fps)) {
         delay_num = 1;
         delay_den = ctx->default_fps;
     }