avformat/rmdec: check that buf if completely filled

Fixes: use of uninitialized value
Fixes: 70988/clusterfuzz-testcase-minimized-ffmpeg_dem_IVR_fuzzer-5298245077630976

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 9578c135d0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rmdec.c

@@ -189,7 +189,8 @@ static int rm_read_audio_stream_info(AVFormatContext *s, AVIOContext *pb,
         st->codecpar->channels = avio_rb16(pb);
         if (version == 5) {
             ast->deint_id = avio_rl32(pb);
-            avio_read(pb, buf, 4);
+            if (avio_read(pb, buf, 4) != 4)
+                return AVERROR_INVALIDDATA;
             buf[4] = 0;
         } else {
             AV_WL32(buf, 0);