fix crash when apply a non-edit of nicknames

[Amit Chaudhuri]

As reported by Henrik, there was a crash lurking in edit_dc_nickname code.
Invoking the dialog and selecting apply without changes exposed it neatly.

This is a one line fix.

Signed off by Amit Chaudhuri <amit.k.chaudhuri at gmail.com>

Looking at some other improvements, but they will come separately once
cleaned up.
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