avformat/au: Check for EOF in au_read_annotation()

Fixes: Timeout (too looong -> 1 ms)
Fixes: 26366/clusterfuzz-testcase-minimized-ffmpeg_dem_SDX_fuzzer-5655584843759616
Fixes: 26391/clusterfuzz-testcase-minimized-ffmpeg_dem_ALP_fuzzer-5484026133217280

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

libavformat/au.c

@@ -84,6 +84,8 @@ static int au_read_annotation(AVFormatContext *s, int size)
     av_bprint_init(&bprint, 64, AV_BPRINT_SIZE_UNLIMITED);
 
     while (size-- > 0) {
+        if (avio_feof(pb))
+            return AVERROR_EOF;
         c = avio_r8(pb);
         switch(state) {
         case PARSE_KEY: