Fix incorrect mate score.

Current master 648c7ec will generate an
incorrect mate score for:

```
setoption name Hash value 8
setoption name Threads value 1
position fen 8/1p2KP2/1p4q1/1Pp5/2P5/N1Pp1k2/3P4/1N6 b - - 76 40
go depth 49
```
even though the position is a draw. Generally, SF tries to display only
proven mate scores, so this is a bug.

This was posted http://www.talkchess.com/forum3/viewtopic.php?f=2&t=72166
by Uri Blass, with the correct analysis that this must be related to the
50 moves draw rule being ignored somewhere.

Indeed, this is possible as positions and there eval are stored in the TT,
without reference to the 50mr counter. Depending on the search path followed
a position can thus be mate or draw in the TT (GHI or Graph history interaction).
Therefore, to prove mate lines, the TT content has to be used with care. Rather
than ignoring TT content in general or for mate scores (which impact search or
mate finding), it is possible to be more selective. In particular, @WOnder93
suggested to only ignore the TT if the 50mr draw ply is closer than the mate
ply. This patch implements this idea, by clamping the eval in the TT to
+-VALUE_MATED_IN_MAX_PLY. This retains the TTmove, but causes a research of
these lines (with the current 50mr counter) as needed.

This patch hardly ever affects search (as indicated by the unchanged
bench), but fixes the testcase. As the conditions are very specific,
also mate finding will almost never be less efficient (testing welcome).

It was also shown to pass STC and LTC non-regression testing, in a form
using if/then/else instead of ternary operators:

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 93605 W: 15346 L: 15340 D: 62919
http://tests.stockfishchess.org/tests/view/5db45bb00ebc5908127538d4

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 33873 W: 7359 L: 7261 D: 19253
http://tests.stockfishchess.org/tests/view/5db4c8940ebc5902d6b146fc

closes #2370

Bench: 4362323

src/search.cpp

@@ -150,7 +150,7 @@ namespace {
   Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
 
   Value value_to_tt(Value v, int ply);
-  Value value_from_tt(Value v, int ply);
+  Value value_from_tt(Value v, int ply, int r50c);
   void update_pv(Move* pv, Move move, Move* childPv);
   void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
   void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
@@ -661,7 +661,7 @@ namespace {
     excludedMove = ss->excludedMove;
     posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
     tte = TT.probe(posKey, ttHit);
-    ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
+    ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
     ttMove =  rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
             : ttHit    ? tte->move() : MOVE_NONE;
     ttPv = PvNode || (ttHit && tte->is_pv());
@@ -899,7 +899,7 @@ namespace {
         search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
 
         tte = TT.probe(posKey, ttHit);
-        ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
+        ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
         ttMove = ttHit ? tte->move() : MOVE_NONE;
     }
 
@@ -1344,7 +1344,7 @@ namespace {
     // Transposition table lookup
     posKey = pos.key();
     tte = TT.probe(posKey, ttHit);
-    ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
+    ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
     ttMove = ttHit ? tte->move() : MOVE_NONE;
     pvHit = ttHit && tte->is_pv();
 
@@ -1530,11 +1530,11 @@ namespace {
   // from the transposition table (which refers to the plies to mate/be mated
   // from current position) to "plies to mate/be mated from the root".
 
-  Value value_from_tt(Value v, int ply) {
+  Value value_from_tt(Value v, int ply, int r50c) {
 
     return  v == VALUE_NONE             ? VALUE_NONE
-          : v >= VALUE_MATE_IN_MAX_PLY  ? v - ply
-          : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
+          : v >= VALUE_MATE_IN_MAX_PLY  ? VALUE_MATE - v > 99 - r50c ? VALUE_MATE_IN_MAX_PLY  : v - ply
+          : v <= VALUE_MATED_IN_MAX_PLY ? VALUE_MATE + v > 99 - r50c ? VALUE_MATED_IN_MAX_PLY : v + ply : v;
   }