Validate hash match in TT lookup

Gain: 16.0 +/- 8.6, LOS: 100.0 %

src/search/negamax.rs

@@ -49,7 +49,6 @@ pub fn negamax(
     if let Some(entry) = entry {
         if alpha == beta - 1
             && plies > 0
-            && entry.hash == game.hash
             && entry.depth >= depth
             && entry.node_type.cutoff_eligible(entry.score, alpha, beta)
         {

src/search/transposition_table.rs

@@ -24,9 +24,17 @@ impl TranspositionTable {
     }
 
     pub fn lookup(&self, zobrist_hash: ZobristHash) -> Option<TTEntry> {
-        self.positions
+        let entry = self
+            .positions
             .get((zobrist_hash.0 & (self.size - 1)) as usize)
-            .and_then(|entry| *entry)
+            .and_then(|entry| *entry);
+
+        if let Some(entry) = entry {
+            if entry.hash == zobrist_hash {
+                return Some(entry);
+            }
+        }
+        None
     }
 
     pub fn insert(&mut self, entry: TTEntry) {