What used to create sorted range automatically in FID_SEARCH_SEARCHSET
handler in SwView::ExecSearch, over time turned into unsorted. this
prevents using MergeRange on the sets created from the range list; see
https://ci.libreoffice.org/job/gerrit_linux_clang_dbgutil/24448/consoleFull

> svl/source/items/itemset.cxx:641:
> void SfxItemSet::MergeRange(sal_uInt16, sal_uInt16):
> Assertion `!pRange[2] || (pRange[2] > pRange[1] && pRange[2] - pRange[1] > 1)' failed.

So let's just use MergeRange in the SwView::ExecSearch itself, which
would ensure early detection if the initial list gets unsorted.

Change-Id: Id0232190f850e3feb463adfcb3153eafbaad7d9b
Reviewed-on: https://gerrit.libreoffice.org/66607Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>