Fix syzygy en passant issue

v = value without ep capture being considered
v1 = value of the ep capture

The correct logic is:
if without e.p. capture we are losing, and the value of e.p is either draw, or win or "loss, but 50 move rule saves us", then we should use the value of ep capture.

Credit and thanks to syzygy1 and lantonov !

No functional change (except with syzygy bases)

Resolves #415
Resolves #394

src/syzygy/tbprobe.cpp

@@ -624,7 +624,7 @@ int Tablebases::probe_dtz(Position& pos, int *success)
       if (v1 >= 0)
         v = v1;
     } else if (v < 0) {
-      if (v1 >= 0 || v1 < 100)
+      if (v1 >= 0 || v1 < -100)
         v = v1;
     } else if (v > 100) {
       if (v1 > 0)