rhbz#1072607: (hopefully) fix crash in SvxRuler::MouseMove()

Not sure why mpParaItem is 0 here, add a check like in every other
SvxRuler method.

(regression from 47a77d7d)

Change-Id: Ic11a4d0c9b6ca217cd59d1d3f6789592996d2c1d

svx/source/dialog/svxruler.cxx

@@ -581,6 +581,9 @@ void SvxRuler::MouseMove( const MouseEvent& rMEvt )
     {
         case RULER_TYPE_INDENT:
         {
+            if (!mpParaItem.get())
+                break;
+
             long nIndex = aSelection.nAryPos + INDENT_GAP;
 
             long nIndentValue = 0.0;