Fix the Windows build broken by revision 113846.

Fix the Windows build broken by revision 113846.

 src/window.c (Fwindow_margins): Return nil when there's no marginal
 area, as per the documented API.
 src/w32term.c (x_scroll_bar_create): Use ALLOCATE_PSEUDOVECTOR, not
 Fmake_vector, as scroll bar's struct members are not all Lisp
 objects now.  This avoids crashes in GC.
 src/w32term.h (struct scroll_bar): Convert fringe_extended_p to a
 bool, so its address could be taken.