fixed clang performance in lz4_fast

fixed clang performance in lz4_fast

The simple change from
`matchIndex+MAX_DISTANCE < current`
towards
`current - matchIndex > MAX_DISTANCE`

is enough to generate a 10% performance drop under clang.
Quite massive.
(I missed as my eyes were concentrated on gcc performance at that time).

The second version is more robust, because it also survives a situation where
`matchIndex > current`
due to overflows.

The first version requires matchIndex to not overflow.
Hence were added `assert()` conditions.

The only case where this can happen is with dictCtx compression,
in the case where the dictionary context is not initialized before loading the dictionary.
So it's enough to always initialize the context while loading the dictionary.