avformat/swfdec: Check outlen before allocation

Fixes: Timeout (too long -> 241ms)
Fixes: 29083/clusterfuzz-testcase-minimized-ffmpeg_dem_SWF_fuzzer-6273684478230528

The source of the magic number is
A very quick simulation of the best case compression for "compress"
below is not nice written code as i did not expect I or anyone else
would ever see it again

I would have preferred some nicer expression or course, but thats
what it seems to be asymptotically. For smaller amounts of data a
tighter bound is possible but i saw no nice way to consider that
and it seems also overkill to try to do it more fine grained for
just this

main(){
    int64_t bits = 0;
    int bank = 256;
    int bitbank = 8;
    for(unsigned i = 0; i<1024*1024*1024*4U-100000;) {
        int word_size = bank-255;
        i += word_size;
        bits += bitbank;

        if (!(bank & (bank-1)))
            bitbank ++;
        bank++;
        if (bitbank > 16) {
            printf("BEST %f \n", 8.0 * i / bits );
            bank = 256;
            bitbank = 8;
        }
    }
}

above assumes i remembered correctly how the algorithm works but the
value was close to what actual compession of zeros gave

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

libavformat/swfdec.c

@@ -377,6 +377,9 @@ static int swf_read_packet(AVFormatContext *s, AVPacket *pkt)
             ff_dlog(s, "bitmap: ch=%d fmt=%d %dx%d (linesize=%d) len=%d->%ld pal=%d\n",
                     ch_id, bmp_fmt, width, height, linesize, len, out_len, colormapsize);
 
+            if (len * 17373LL < out_len)
+                goto bitmap_end_skip;
+
             zbuf = av_malloc(len);
             if (!zbuf) {
                 res = AVERROR(ENOMEM);