nautilus-window: Initialize NautilusDragSelectionItem 'file' member

This was probably a slip from commit 916bfc628001b399e53a0d26dac1eeb176734316

The 'file' member was not being initialized, and this caused the
subsequent call to check_same_fs() to receive its dropped_file
parameter as NULL, so returning as not being on same fs, and
so choosing GDK_ACTION_COPY instead of GDK_ACTION_MOVE.

Fixes bug 710198