mp3on4: require a minimum framesize.

If bufsize < headersize, init_get_bits() will be called with a negative
number, causing it to fail and any subsequent call to get_bits() will
crash because it reads from a NULL pointer.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/mpegaudiodec.c

@@ -1917,6 +1917,10 @@ static int decode_frame_mp3on4(AVCodecContext *avctx, void *data,
         m     = s->mp3decctx[fr];
         assert(m != NULL);
 
+        if (fsize < HEADER_SIZE) {
+            av_log(avctx, AV_LOG_ERROR, "Frame size smaller than header size\n");
+            return AVERROR_INVALIDDATA;
+        }
         header = (AV_RB32(buf) & 0x000fffff) | s->syncword; // patch header
 
         if (ff_mpa_check_header(header) < 0) // Bad header, discard block