Dev fix: The API chaining was a little broken if the inner function

returned undefined, null etc.

- The base issue was in the instanceof check which has a priority error,
  it was:
    ! obj instanceof _Api
  which is the same as:
   (! obj) instanceof _Api
  which is rubbish.

  We want:
    ! (obj instanceof _Api)
  but there is of course a wrapper function, so that needs ot be taken
  into account as well. The new logic does just that.

.datatables-commit-sync

@@ -1 +1 @@
-39706899b20912b09581cc44ef838d469ab4e2fa
+c45d7a00a9cb9889d6782276fb21e8c7d2f121e8

media/js/jquery.dataTables.js

@@ -6657,14 +6657,15 @@
 	
 	_Api.extend = function ( scope, obj, ext )
 	{
-		if ( ! obj instanceof _Api ) {
+		// Only extend API instances and static properties of the API
+		if ( ! obj || ( ! (obj instanceof _Api) && ! obj.__dt_wrapper ) ) {
 			return;
 		}
 	
 		var
 			i, ien,
 			j, jen,
-			struct,
+			struct, inner,
 			methodScoping = function ( fn, struc ) {
 				return function () {
 					var ret = fn.apply( scope, arguments );
@@ -6677,14 +6678,14 @@
 	
 		for ( i=0, ien=ext.length ; i<ien ; i++ ) {
 			struct = ext[i];
+			inner = obj[ struct.name ];
 	
 			// Value
-			if ( typeof struct.val === 'function' ) {
-				obj[ struct.name ] = methodScoping( struct.val, struct );
-			}
-			else {
-				obj[ struct.name ] = struct.val;
-			}
+			inner = typeof struct.val === 'function' ?
+				methodScoping( struct.val, struct ) :
+				struct.val;
+	
+			inner.__dt_wrapper = true;
 	
 			// Property extension
 			_Api.extend( scope, obj[ struct.name ], struct.propExt );