Increase TT size limit to 8 GB

We had an overflow due to use an integer for hash size,
now we use a size_t as we should, so we can increase to
an higher limit.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/tt.cpp

@@ -53,11 +53,9 @@ TranspositionTable::~TranspositionTable() {
 /// TranspositionTable::set_size sets the size of the transposition table,
 /// measured in megabytes.
 
-void TranspositionTable::set_size(unsigned mbSize) {
+void TranspositionTable::set_size(size_t mbSize) {
 
-  assert(mbSize >= 4 && mbSize <= 2048);
-
-  unsigned newSize = 1024;
+  size_t newSize = 1024;
 
   // We store a cluster of ClusterSize number of TTEntry for each position
   // and newSize is the maximum number of storable positions.