"JS error after using mousewheel without having moved the mouse before."

Update MouseWheel handler to use center of map if no mousemovement has 
been seen yet. (Closes #882)


git-svn-id: http://svn.openlayers.org/trunk/openlayers@4042 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

tests/list-tests.html

@@ -72,6 +72,7 @@
     <li>Control/test_Permalink.html</li>
     <li>Control/test_Scale.html</li>
     <li>test_Handler.html</li>
+    <li>Handler/test_MouseWheel.html</li>
     <li>Handler/test_Drag.html</li>
     <li>Handler/test_Point.html</li>
     <li>Handler/test_Path.html</li>