add safety check to get_slot (this appears to be part of the error andrew is getting - possibly the victim of mem corruption)

This was SVN commit r3151.

source/lib/res/file/vfs_tree.cpp

@@ -166,6 +166,7 @@ public:
 	T* get_slot(Key key)
 	{
 		u32 hash = Hash(key);
+		debug_assert(max_entries != 0);	// otherwise, mask will be incorrect
 		const uint mask = max_entries-1;
 		T* p;
 		for(;;)