Crash when two different en passant captures are possible

[bennedik]

There is a crash in EGTB.cs if there are two different en passant captures possible by two pawns on two different files, capturing the same pawn en passant.

This is because the following code is trying to execute the capture a second time, without resetting the first capture, in the method egtb_get_dtm:

                            /* execute capture */
                            xs[i] = epsq;
                            removepiece(ref ys, ref yp, j);

For example in the following position:
8/8/8/8/4p1p1/8/4KPk1/8 w - - 0 6

After the move f4, there are two en passant captures possible.