avformat/flacdec: Check avio_read result when reading flac block header.

Return AVERROR_INVALIDDATA if all four bytes aren't present.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 95bde499)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/flacdec.c

@@ -49,7 +49,8 @@ static int flac_read_header(AVFormatContext *s)
 
     /* process metadata blocks */
     while (!avio_feof(s->pb) && !metadata_last) {
-        avio_read(s->pb, header, 4);
+        if (avio_read(s->pb, header, 4) != 4)
+            return AVERROR(AVERROR_INVALIDDATA);
         flac_parse_block_header(header, &metadata_last, &metadata_type,
                                    &metadata_size);
         switch (metadata_type) {