xen/intr: xen_intr_bind_isrc() always set handle

Previously the upper layer handle was being set before the last
potential error condition. The reasoning appears to have been it was
assumed invalid in case of an error being returned. Now ensure it is
invalid until just before a successful return.

Fixes: 76acc41fb7c7 ("Implement vector callback for PVHVM and unify event channel implementations")
Fixes: 6d54cab1fe7d ("xen: allow to register event channels without handlers")
Reviewed by: royger