impurecord.c: corrected small bug in remove_impucontact_from_list()

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#### Description <!-- Describe your changes in detail -->

corrected 2 small bugs in function remove_impucontact_from_list: nullify tail->next if tail removed in remove mid-list the pointer to previous contact was not set correctly

These issues were brought up by https://github.com/thulasizwe-n at another pull request: https://github.com/kamailio/kamailio/pull/1547#issuecomment-395767031

You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/1562

-- Commit Summary --

* ims_usrloc_scscf:bugfix erraneous unlinking of deleted contact

-- File Changes --

M src/modules/ims_usrloc_scscf/impurecord.c (3)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/1562.patch https://github.com/kamailio/kamailio/pull/1562.diff