Fixed Valgrind warnings

Running cockatrice in valgrind i didn't found any bad memory leak
(good!), but only a few jumps depending on uninitialized variables.
Even if the possibility of crashes are remote, it's nice to sort them
out.

common/server_response_containers.cpp

@@ -15,7 +15,7 @@ GameEventStorageItem::~GameEventStorageItem()
 }
 
 GameEventStorage::GameEventStorage()
-    : gameEventContext(0)
+    : gameEventContext(0), privatePlayerId(0)
 {
 }