chooser: avoid free of uninitialized pointer in allocation failure cleanup

Resolves issue found by Oracle Parfait static analyzer:

Error: Uninitialised memory
   Uninitialised memory [uninitialised-mem] (CWE 456):
      Possible access to uninitialised memory referenced by 'new->fullname'
      Memory 'new->fullname' accessed at byte offset '8'
        at line 357 of app/xdm/chooser/chooser.c in function 'AddHostname'.
        Write does not overlap at line 355
          new allocated at line 329

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
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