Bulletproofing the tree to not divide by 0 in the case of scrolling or

asking the row count of a 0-row tree.  Bug 48306.  r=syd.

layout/xul/base/src/nsXULTreeOuterGroupFrame.cpp

@@ -921,7 +921,7 @@ nsXULTreeOuterGroupFrame::EnsureRowIsVisible(PRInt32 aRowIndex)
 void
 nsXULTreeOuterGroupFrame::ScrollToIndex(PRInt32 aRowIndex)
 {
-  if ( aRowIndex < 0 )
+  if (( aRowIndex < 0 ) || (mRowHeight == 0))
     return;
     
   PRInt32 newIndex = aRowIndex;

layout/xul/base/src/nsXULTreeOuterGroupFrame.h

@@ -142,7 +142,8 @@ public:
   nscoord GetYPosition();
   nscoord GetAvailableHeight();
   NS_IMETHOD GetNumberOfVisibleRows(PRInt32 *aResult) {
-    *aResult=GetAvailableHeight() / mRowHeight; return NS_OK;
+    *aResult=(mRowHeight ? GetAvailableHeight() / mRowHeight : 0);
+    return NS_OK;
   }
   NS_IMETHOD GetIndexOfFirstVisibleRow(PRInt32 *aResult) {
     *aResult=mCurrentIndex; return NS_OK;