offset: make sure there is a parent node to work on while calculating…

… scroll offsets (#2073)

src/offset.js

@@ -55,7 +55,7 @@ jQuery.fn.offset = function() {
 			}
 
 			// Get parent scroll offsets
-			while ( parent.tagName && !/^body|html$/i.test(parent.tagName) ) {
+			while ( parent && parent.tagName && !/^body|html$/i.test(parent.tagName) ) {
 				// Remove parent scroll UNLESS that parent is inline or a table to work around Opera inline/table scrollLeft/Top bug
 				if ( !/^inline|table.*$/i.test(jQuery.css(parent, "display")) )
 					// Subtract parent scroll offsets