fix crash when more than one user exists and we pre-select one
Review Request #106006 - Created Aug. 13, 2012 and submitted
since df12892c7a8b1475b6590186f590b349f52a54d4 the current user is pre-selected in the KComboBox when more than one user is present. This will crash the agent as the user identity will be NULL because the item change signal isn't connected at this point. This patch: a) fixes that as it sets the identity after the dialog is initialized b) pre-selects the password field which should be the intended behaviour if we're already pre-selecting a user.
I verified that this works by allowing muon to install packages with me pre-selected on kubuntu.