Fix for bug 46040. We need to dismiss the popup menu when the user press the tab key. R=mscott

Aso, I have incorporated a patch submitted by Matthew Cline (bug 42582)

xpfe/components/autocomplete/resources/content/autocomplete.xml

@@ -272,6 +272,9 @@
 			},
 
 			keyNavigation: function(me, event, popup) {
+        if (me.lastResults == null)
+          return;
+
 				var inputElement = document.getAnonymousNodes(me)[0].firstChild;
 				if (event.keyCode == 38 || event.keyCode == 40)
 				{
@@ -356,9 +359,15 @@
 					    me.autoCompleteTimer = 0;
 				    }
 						me.privatefunc.closePopupMenu(me);
-						me.lastResults.searchString='';
+            if (me.lastResults != null)
+						  me.lastResults.searchString='';
 						return;
 
+          case 9: /*vk_tab*/
+						if (popup)
+              me.privatefunc.closePopupMenu(me);
+            return;
+
 					case 13 /*vk_return*/:
 				    if (me.autoCompleteTimer) {
 					    clearTimeout(me.autoCompleteTimer);