Small reformat in evaluate threats (non functional)

Small reformat in evaluate threats (non functional)

When evaluating threat by safe pawn and pawn push the same expression is used.

STC
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 19444 W: 4540 L: 4309 D: 10595
http://tests.stockfishchess.org/tests/view/5b5a6e150ebc5902bdb8c5c0

Closes https://github.com/official-stockfish/Stockfish/pull/1709

No functional change.

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Comments by Stéphane Nicolet:

I don't measure any speed-up on my system, with two parallel benches at depth 22:

Total time (ms) : 74989
Nodes searched : 144830258
Nodes/second : 1931353
master

Total time (ms) : 75341
Nodes searched : 144830258
Nodes/second : 1922329
testedpatch

And anyway, like Stefan Geschwentner, I don't think that a 0.3% speed-up would
be enough to pass a [0..5] LTC test -- as a first approximation, we have this
rule of thumb that 1% speed-up gives about 1 Elo point.

However, considering the facts that the reformatting by itself is interesting,
that this is your first green test and that you played by the rules by running
the SPRT[0..5] test before opening the pull request, I will commit the change.
I will only take the liberty to change the occurrences of safe in lines 590 and
591 to b, to make the code more similar to lines 584 and 585.

So approved, and congrats :-)