avcodec/htmlsubtitles: Be a bit more picky on syntax

This reduces the number of strstr() calls per byte
This diasalows empty tags like '< >' as well as '<' in tags like '<ab<cd<<ef>'

Fixes timeout
Fixes: 1817/clusterfuzz-testcase-minimized-5104230530547712

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