avformat/apngdec: Check fcTL chunk length when reading header

Reading the header terminates when an fcTL chunk is encountered in which
case read_header returned success without checking the length of said
chunk. Yet when read_packet processes this chunk, it checks for the
length to be 26 and errors out otherwise. So do so when reading the header,
too.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/apngdec.c

@@ -226,7 +226,7 @@ static int apng_read_header(AVFormatContext *s)
                                     ctx->num_frames, ctx->num_play);
             break;
         case MKTAG('f', 'c', 'T', 'L'):
-            if (!acTL_found) {
+            if (!acTL_found || len != 26) {
                ret = AVERROR_INVALIDDATA;
                goto fail;
             }