Fix abort from stl debug iterators' invalid access.

Triggered by fdo#43725, incrementing an invalid iterator bombs -
though this seems a corner case, depends on whether one considers
"+= 0" as incrementing or not.

basegfx/source/polygon/b2dpolypolygon.cxx

@@ -84,6 +84,7 @@ public:
         {
             // add nCount copies of rPolygon
             PolygonVector::iterator aIndex(maPolygons.begin());
+            if( nIndex )
                 aIndex += nIndex;
             maPolygons.insert(aIndex, nCount, rPolygon);
         }
@@ -93,6 +94,7 @@ public:
     {
         // add all polygons from rPolyPolygon
         PolygonVector::iterator aIndex(maPolygons.begin());
+        if( nIndex )
             aIndex += nIndex;
         maPolygons.insert(aIndex, rPolyPolygon.begin(), rPolyPolygon.end());
     }

basegfx/source/polygon/b3dpolypolygon.cxx

@@ -83,6 +83,7 @@ public:
         {
             // add nCount copies of rPolygon
             PolygonVector::iterator aIndex(maPolygons.begin());
+            if( nIndex )
                 aIndex += nIndex;
             maPolygons.insert(aIndex, nCount, rPolygon);
         }
@@ -92,6 +93,7 @@ public:
     {
         // add all polygons from rPolyPolygon
         PolygonVector::iterator aIndex(maPolygons.begin());
+        if( nIndex )
             aIndex += nIndex;
         maPolygons.insert(aIndex, rPolyPolygon.begin(), rPolyPolygon.end());
     }