Fix HideMapNamePopUpWindow possible overflow

[DizzyEggg]

Followup to #1976

We found the cause of the glitching map graphics. It was GetMapNamePopUpWindowId passing 0xFF as valid windowId, which in turn modifies the data directly below gWindows, which happened to be gHeap on some compiler/linker versions. In vanilla this goes unnoticed.

[GriffinRichards]

It's too bad they didn't have a single point of reference for gWindows, it'd be nice to do this kind of check there (because I wouldn't be surprised if there are some other places this is happening quietly). But by my count there's 23 functions that access gWindows with an arbitrary window id, so this is definitely the less intrusive fix.