DropTarget: Fix critical if `reject()` drop before `::enter`

Fixes #5922 (closed)

The docs of Gtk.DropTarget::accept say this:

If the decision whether the drop will be accepted or rejected depends on the data, [::accept] should return TRUE, [:preload] should be set and the value should be inspected via the ::notify:value signal, calling gtk_drop_target_reject() if required.

But this pattern causes a CRITICAL, given these steps:

• Create a DragSource and DropTarget
• Keep the default ::accept handler and set :preload to TRUE
• Connect to notify::value and therein call DropTarget.reject()
• CRITICAL at DropTarget.enter()→Drop.get_actions() on NULL instance

We should let the documented case work without a CRITICAL or worse, null deref. And per @otte on the bug we should bail earlier, before ::enter & setting GTK_STATE_FLAG_DROP_ACTIVE if reject()ed, unlike at present.

This patch fixes it, and does so unconditional on whether we preload, in case notify::*drop may call reject(), though they probably shouldn’t