Fix #11969. Never a null moment when checking siblings.

src/traversing.js

@@ -158,7 +158,7 @@ function isDisconnected( node ) {
 function sibling( cur, dir ) {
 	do {
 		cur = cur[ dir ];
-	} while ( cur.nodeType !== 1 );
+	} while ( cur && cur.nodeType !== 1 );
 
 	return cur;
 }

test/unit/traversing.js

@@ -467,11 +467,12 @@ test("parentsUntil([String])", function() {
 });
 
 test("next([String])", function() {
-	expect(4);
+	expect(5);
 	equal( jQuery("#ap").next()[0].id, "foo", "Simple next check" );
 	equal( jQuery("#ap").next("div")[0].id, "foo", "Filtered next check" );
 	equal( jQuery("#ap").next("p").length, 0, "Filtered next check, no match" );
 	equal( jQuery("#ap").next("div, p")[0].id, "foo", "Multiple filters" );
+	equal( jQuery("body").next().length, 0, "Simple next check, no match" );
 });
 
 test("prev([String])", function() {