Bug 493037 - "aNumLines must be non-zero" assertion hit on mouse wheel input. r=emaijala.

widget/src/windows/nsWindow.cpp

@@ -5012,6 +5012,9 @@ PRBool nsWindow::OnMouseWheel(UINT msg, WPARAM wParam, LPARAM lParam, PRBool& ge
     }
   }
 
+  if (!scrollEvent.delta)
+    break;
+
   scrollEvent.isShift   = IS_VK_DOWN(NS_VK_SHIFT);
   scrollEvent.isControl = IS_VK_DOWN(NS_VK_CONTROL);
   scrollEvent.isMeta    = PR_FALSE;