Fix a coverity scan warning

Coverity scan warns about uninitialized 'sf' argument when
calling probe(). Actually it is a false positive because
argument is passed by reference and assigned inside
probe(). Nevertheless it is a hint that fucntion signature
is a bit tricky, so rewrite it in a more conventional way,
assigning 'sf' from probe() return value.

No functional change.

src/endgame.h

@@ -113,8 +113,8 @@ class Endgames {
 public:
   Endgames();
 
-  template<typename T> T* probe(Key key, T** eg)
-  { return *eg = map<T>().count(key) ? map<T>()[key].get() : nullptr; }
+  template<typename T, typename E = EndgameBase<T>> E* probe(Key key)
+  { return map<E>().count(key) ? map<E>()[key].get() : nullptr; }
 };
 
 #endif // #ifndef ENDGAME_H_INCLUDED