avformat/rmdec: Check for EOF in index packet reading

Fixes: Timeout(>10sec -> 1ms)
Fixes: 27284/clusterfuzz-testcase-minimized-ffmpeg_dem_RM_fuzzer-6304211110985728

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ebf4bc629e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rmdec.c

@@ -455,6 +455,8 @@ static int rm_read_index(AVFormatContext *s)
         }
 
         for (n = 0; n < n_pkts; n++) {
+            if (avio_feof(pb))
+                return AVERROR_INVALIDDATA;
             avio_skip(pb, 2);
             pts = avio_rb32(pb);
             pos = avio_rb32(pb);