Fix crash when unmounting volumes

[yut23]

This was a use-after-free of monitor, which gets freed by vfs_dir_finalize():

spacefm/src/vfs/vfs-dir.c

Lines 228 to 233 in e6f2918

	if ( dir->monitor )
	{
	vfs_file_monitor_remove( dir->monitor,
	vfs_dir_monitor_callback,
	dir );
	}

I'm not sure why the VFSDir gets destroyed at that point, but it was probably some internal GLib change, as usual.

Fixes #819.

[morfikov]

@mati75
I can confirm that the patch works well.

[biopsin]

Tested and segfault on unmounting usb drives or other with a open tab no longer happens, thank you!