quartz: NSWindow NULL check in get_scale_factor

Sending backingScaleFactor to a NULL NSWindow will silently give the
value 0 for the scale factor, causing insidious divide-by-zero bugs down
the line. This checks if the NSWindow is NULL first, as seems to happen
throughout the rest of the file.

Note that I don't have a hi-DPI OS X machine to test this on, though.

https://bugzilla.gnome.org/show_bug.cgi?id=738338