entry: Handle no-window events in gtk_entry_event

Andrea Azzarone requested to merge azzaronea/gtk:fix-issue-1366 into gtk-3-24

gtk_entry_event's goal is to detect if a specific event concerns one of the two entry icons. It can happen that this function is called during initialization and/or before the entry is realized. In this case the entry icons (and the event) will not yet have an associated window. The code should consider the aforementioned situation and avoid matching a icon and an event with no associated windows.

Fixes: #1366

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