Minor improvement in eval of unstoppable pawns promoting one ply apart.

Marco's new code for evaluating two unstoppable passed pawns where
one pawn promotes a single ply before the other tried to detect
cases where the pawn that promotes first could immediately capture
the pawn that promotes a ply later, but didn't work in cases where
the two pawns are on the same file. An example of this is the
following position:

8/8/3K4/2P5/2p5/3k4/8/8 w - -

With the new code, such positions are handled correctly.