avformat/apetag: Check APETAGEX

Fixes: Use of uninitialized value
Fixes: 71074/clusterfuzz-testcase-minimized-ffmpeg_IO_DEMUXER_fuzzer-5697034877730816

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

libavformat/apetag.c

@@ -121,7 +121,8 @@ int64_t ff_ape_parse_tag(AVFormatContext *s)
 
     avio_seek(pb, file_size - APE_TAG_FOOTER_BYTES, SEEK_SET);
 
-    avio_read(pb, buf, 8);     /* APETAGEX */
+    if(avio_read(pb, buf, 8) != 8)     /* APETAGEX */
+        return 0;
     if (strncmp(buf, APE_TAG_PREAMBLE, 8)) {
         return 0;
     }