avformat/hls: Fix DoS due to infinite loop

Fixes: loop.m3u

The default max iteration count of 1000 is arbitrary and ideas for a better solution are welcome

Found-by: Xiaohei and Wangchu from Alibaba Security Team

Previous version reviewed-by: Steven Liu <lingjiujianke@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>