[Quartz Bug 663182] NSImage throws an exception from _gtk_quartz_create_image_from_pi...

[Quartz Bug 663182] NSImage throws an exception from _gtk_quartz_create_image_from_pixbuf()"

If a zero-sized NSImage calls lockfocus it throws an exception. Make sure that the image isn't zero-sized before calling lockfocus, and log a warning and return NULL if it is zero-sized.

Have the quartz version of gtk_drag_begin_idle return FALSE if the returned image is NULL.