The big NULL replacement party part 1.

This is meant to be a safer alternative since
NULL can often be 0. Let's not rely on that.

And yes, I know this is a lot of files. This is
a safer thing to do in big commits vs for loops.

src/MemoryCardManager.cpp

@@ -298,7 +298,7 @@ MemoryCardManager::~MemoryCardManager()
 	// Unregister with Lua.
 	LUA->UnsetGlobal( "MEMCARDMAN" );
 
-	ASSERT( g_pWorker != NULL );
+	ASSERT( g_pWorker != nullptr );
 	SAFE_DELETE(g_pWorker);
 
 	FOREACH_PlayerNumber( pn )