Whoopsee... managed to introduce a bug when changing is_null() to === NULL for performance reasons, and reversed the logic of the test..... this is the fix

git-svn-id: https://phpexcel.svn.codeplex.com/svn/trunk@86204 2327b42d-5241-43d6-9e2a-de5ac946f064

Classes/PHPExcel/Calculation.php

@@ -3222,7 +3222,7 @@ class PHPExcel_Calculation {
 					}
 				} else {
 //					echo 'Reference is a single Cell<br />';
-					if ($pCell !== NULL) {
+					if ($pCell === NULL) {
 //						We can't access the cell, so return a REF error
 						$cellValue = PHPExcel_Calculation_Functions::REF();
 					} else {