dash: Make sure _clearDragPlaceholder() resets _dragPlaceholderPos

dash: Make sure _clearDragPlaceholder() resets _dragPlaceholderPos

The function currently only resets the placeholder position if
there is a placeholder; this is not necessarily true, as the
placeholder may be reset outside _clearDragPlaceholder().
If this happens, the placeholder will temporarily stop working
for the "old" position (and permanently if it's the only position).
Just reset the position unconditionally.

https://bugzilla.gnome.org/show_bug.cgi?id=684618