Fix leak in SwFieldPage::InsertField, where it interacts badly with
SwFieldMgr::UpdateCurField

The awkward
   return std::unique_ptr<SwField>(pTmp.release());
code is because clang 3.8.0 doesn't seem able to do auto-upcast of
std::unique_ptr.

Change-Id: I05b748e9cda145fd972a01bfa343239df32368b9
Reviewed-on: https://gerrit.libreoffice.org/57197
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>