Eliminate EPPlus 'null reference' exception

Release note: No

Developer Comment:

Eliminate EPPlus 'null reference' exception.

Change-Id: I134e32144fd02acee483926b668ded51e4666019

EPPlus/ExcelNamedRangeCollection.cs

diff --git a/EPPlus/ExcelNamedRangeCollection.cs b/EPPlus/ExcelNamedRangeCollection.cs
index 537a7db..2cad528 100644
--- a/EPPlus/ExcelNamedRangeCollection.cs
+++ b/EPPlus/ExcelNamedRangeCollection.cs
@@ -88,7 +88,7 @@
                 if ((0 <= index) && (index < _list.Count))
                 {
                     ExcelNamedRange listItem = _list[index];
-                    if ((listItem != null) && (listItem.FullAddress == item.FullAddress))
+                    if ((listItem != null) && (listItem.FullAddress != null) && (listItem.FullAddress == item.FullAddress))
                         return;
 
                     //throw new Exception(string.Format("Name '{0}' is defined in the worksheet more than once. First as '{1}' and second as '{2}'.", Name, listItem.FullAddress, item.FullAddress));