Assign improving only once

Avoid duplicated code after recent commit "Use evaluation trend
to adjust futility margin". We initialize the improving variable
to true in the check case, which allows to avoid redundant code
in the general case.

Tested for speed by snicolet, patch seems about 0.4% faster.

No functional change.

Note: initializing the improving variable to false in the check
case was tested as a functional change, ending yellow in both STC
and LTC. This change is not included in the commit, but it is an
interesting result that could become part of a future patch about
improving or LMR. Reference of the LTC yellow test:
http://tests.stockfishchess.org/tests/view/5aa131560ebc590297cb636e