tdf#91969: Parts of paragraph border is missing

...when border is around more than one paragraph

Regression from:
16f637da

Change-Id: Iea9455dd7cc3c301c3e2d5d7903c98d98e515595
(cherry picked from commit 8d933132)
Reviewed-on: https://gerrit.libreoffice.org/17979Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/core/layout/paintfrm.cxx

@@ -477,13 +477,13 @@ static sal_uInt8 lcl_TryMergeLines(
 {
     double const fMergeGap(properties.nSPixelSzW + properties.nSHalfPixelSzW); // NOT static!
     // A is above/before B
-    if( mergeA.second <= mergeB.second &&
+    if( mergeA.second <= mergeB.first &&
         mergeA.second + fMergeGap >= mergeB.first )
     {
         return 1;
     }
     // B is above/before A
-    else if( mergeB.second <= mergeA.second &&
+    else if( mergeB.second <= mergeA.first &&
              mergeB.second + fMergeGap >= mergeA.first )
     {
         return 2;