bnc#862514: Handle corner case in slide sorter correctly.

We should never return -1 when bIncludeBordersAndGaps is true; but that could
have happened in a corner case:

* export SAL_USE_VCLPLUGIN=gen
* start LibreOffice
* open a presentation with many slides
* resize the window so that it shows the slides _exactly_, no slide is cut
  off in the slide sorter
* point the mouse pointer into the left "Slides" sidebar (slide sorter) so
  that it does not cause any mouse-over effect (somehow between the left
  border, and the slides - there is space that can do this)
* turn the mouse wheel all the way down so that the last slide is at the
  bottom
* turn the mouse wheel up _once_
* turn the mouse wheel down _once_ - it will look like the view does not
  change (the last but one is still at the bottom), but you can see the
  scrollbar jump to the top
* and now you can again start turning the wheel all the way down to repeat
  the procedure :-)

Change-Id: I0253c89513222a1f2eb3263068997d10eea0f130

sd/source/ui/slidesorter/view/SlsLayouter.cxx

@@ -560,11 +560,15 @@ sal_Int32 Layouter::Implementation::GetRowAtPosition (
         // When inside the gap below then nYPosition is not over a page
         // object.
         if (nDistanceIntoGap > 0)
-            nRow = ResolvePositionInGap (
+        {
+            sal_Int32 nResolvedRow = ResolvePositionInGap(
                 nDistanceIntoGap,
                 eGapMembership,
                 nRow,
                 mnVerticalGap);
+            if (!bIncludeBordersAndGaps || nResolvedRow != -1)
+                nRow = nResolvedRow;
+        }
     }
     else if (bIncludeBordersAndGaps)
     {
@@ -600,11 +604,15 @@ sal_Int32 Layouter::Implementation::GetColumnAtPosition (
         // When inside the gap at the right then nXPosition is not over a
         // page object.
         if (nDistanceIntoGap > 0)
-            nColumn = ResolvePositionInGap (
+        {
+            sal_Int32 nResolvedColumn = ResolvePositionInGap(
                 nDistanceIntoGap,
                 eGapMembership,
                 nColumn,
                 mnHorizontalGap);
+            if (!bIncludeBordersAndGaps || nResolvedColumn != -1)
+                nColumn = nResolvedColumn;
+        }
     }
     else if (bIncludeBordersAndGaps)
     {