fix performance issue in scenario #2966 (part 1)

fix performance issue in scenario #2966 (part 1)

When re-using a compression state, across multiple successive compressions,
the state should minimize the amount of allocation and initialization required.

This mostly matters in situations where initialization is an overwhelming task
compared to compression itself.
This can happen when the amount to compress is small,
while the compression state was given the impression that it would be much larger,
aka, streaming mode without providing a srcSize hint.

This lean-initialization optimization was broken in 980f3bbf8354edec0ad32b4430800f330185de6a .

This commit fixes it, making this scenario once again on par with v1.4.9.

Note that this does not completely fix #2966,
since another heavy initialization, specific to row mode,
is also happening (and was not present in v1.4.9).
This will be fixed in a separate commit.