bug#2465: NS Crash (#2465)

[Adrian Robert]

> *** Terminating app due to uncaught exception
> 'NSInternalInconsistencyException', reason: '-[EmacsView(0xe87e430)
> lockFocus] failed with window=0xe892b90, windowNumber=-1, [self
> isHiddenOrHasHiddenAncestor]=0'

This happens when lockFocus is called on a second window before focus  
has been unlocked on the first and the window flushed.

In nsterm.m, these are wrapped by ns_focus and ns_unfocus, and these  
should always be matched.