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