[lz4hc] Improve pattern detection in ext dict

[lz4hc] Improve pattern detection in ext dict

It is important to continue to look backwards if the current pattern
reaches `lowPrefixPtr`. If the pattern detection doesn't go all the
way to the beginning of the pattern, or the end of the pattern it
slows down the search instead of speeding it up.

The slow unit in `round_trip_stream_fuzzer` used to take 12 seconds
to run with -O3, now it takes 0.2 seconds.

Credit to OSS-Fuzz