sw: erroneous vector assignment in SwTable::OldMerge()

+    rLineBoxes = pFndBox->GetLines()[nEnd]->GetBoxes();

This actually assigns the boxes vector of the last line to the first
line, which the previous code didn't do.

Notably after converting the types from boost::ptr_vector to
std::vector<std::unique_ptr>, the assignment produces a several page
error message from both GCC and clang, and infuriatingly neither
compiler was able to tell on which line in tblrwcl.cxx the template
was instantiated, so i had to bisect that.

(regression from be5e4247)

Change-Id: I646e3ca678895480d38ec48f38d720458860a985
(cherry picked from commit 8bf0e60d)
Reviewed-on: https://gerrit.libreoffice.org/18379Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>

sw/source/core/doc/tblrwcl.cxx

@@ -1613,17 +1613,15 @@ bool SwTable::OldMerge( SwDoc* pDoc, const SwSelBoxes& rBoxes,
     _InsULPara aPara( pTableNd, true, true, pLeftBox, pMergeBox, pRightBox, pInsLine );
 
     // Move the overlapping upper/lower Lines of the selected Area
-    _FndBoxes& rLineBoxes = pFndBox->GetLines().front().GetBoxes();
-    for (_FndBoxes::iterator it = rLineBoxes.begin(); it != rLineBoxes.end(); ++it)
+    for (auto & it : pFndBox->GetLines().front().GetBoxes())
     {
-        lcl_Merge_MoveBox(*it, &aPara);
+        lcl_Merge_MoveBox(it, &aPara);
     }
     aPara.SetLower( pInsLine );
     const auto nEnd = pFndBox->GetLines().size()-1;
-    rLineBoxes = pFndBox->GetLines()[nEnd].GetBoxes();
-    for (_FndBoxes::iterator it = rLineBoxes.begin(); it != rLineBoxes.end(); ++it)
+    for (auto & it : pFndBox->GetLines()[nEnd].GetBoxes())
     {
-        lcl_Merge_MoveBox(*it, &aPara);
+        lcl_Merge_MoveBox(it, &aPara);
     }
 
     // Move the Boxes extending into the selected Area from left/right