ZSTD_fast_noDict: Minimize Checks When Writing Hash Table for ip1

ZSTD_fast_noDict: Minimize Checks When Writing Hash Table for ip1

This commit avoids checking whether a hashtable write is safe in two of the
three match-found paths in `ZSTD_compressBlock_fast_noDict_generic`. This pro-
duces a ~0.5% speed-up in compression.

A comment in the code describes why we can skip this check in the other two
paths (the repcode check and the first match check in the unrolled loop).

A downside is that in the new position where we make this check, we have not
yet computed `mLength`. We therefore have to avoid writing *possibly* dangerous
positions, rather than the old check which only avoids writing *actually*
dangerous positions. This leads to a miniscule loss in ratio (remember that
this scenario can only been triggered in very negative levels or under incomp-
ressibility acceleration).