spice-widget: check whether the widget is realized in focus_in_event()
spice-widget: check whether the widget is realized in focus_in_event()

Returning early on focus_in_event(), when widget is not realized, avoids
segfault when running on Windows using GTK3.

Program received signal SIGSEGV, Segmentation fault.
_gdk_windows_has_impl <window=window@entry=0x0> at gdkwindow.c:584
<gdb> bt
 #0  _gdk_window_has_impl (window=window@entry=0x0) at gdkwindow.c:584
 #1  0x70f02821 in gdk_win32_window_get_handle (window=0x0) at
     gdkwindow-win32.c:3459
 #2  0x00c759ef in update_display (display=0x1b18440) at
     spice-widget.c:1297
 #3  0x00c77280 in focus_in_event (widget=0x1b18440, focus=0x1b02b68) at
     spice-widget.c:1462
 #4  0x665727f5 in ?? () from C:\Program Files\VirtViewer
     (GTK3)\bin\libgtk-3-0.dll
 #5  0x00000000 in ?? ()

Fantu/spice-gtk,fgouget/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk