Fix memory leak.

dlls/gamerules.h

@@ -59,6 +59,8 @@ enum
 class CGameRules
 {
 public:
+	virtual ~CGameRules();
+
 	virtual void RefreshSkillData( void );// fill skill data struct with proper values
 	virtual void Think( void ) = 0;// GR_Think - runs every server frame, should handle any timer tasks, periodic events, etc.
 	virtual BOOL IsAllowedToSpawn( CBaseEntity *pEntity ) = 0;  // Can this item spawn (eg monsters don't spawn in deathmatch).

dlls/world.cpp

@@ -474,6 +474,7 @@ void CWorld::Precache( void )
 	if( g_pGameRules )
 	{
 		delete g_pGameRules;
+		g_pGameRules = NULL;
 	}
 
 	g_pGameRules = InstallGameRules();