return error if read failed

Originally committed as revision 17173 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/ffmdec.c

@@ -30,7 +30,8 @@ int64_t ffm_read_write_index(int fd)
     uint8_t buf[8];
 
     lseek(fd, 8, SEEK_SET);
-    read(fd, buf, 8);
+    if (read(fd, buf, 8) != 8)
+        return AVERROR(EIO);
     return AV_RB64(buf);
 }