Fixed a bug in UnloadDIFArray when a file open was not successful.

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@2111 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/fpspreadsheet/uvirtuallayer_ole_helpers.pas

@@ -501,6 +501,11 @@ var
   NextDIFC: SECT;
   Index: SizeUint;
 begin
+  if not Assigned(FDIFArray) then begin
+    //Nothing to be freed.
+    Result:=true;
+    exit;
+  end;
   //The first DIF is an special one and do not need to be
   //written as it is written with the header updates.
   FDIFArray[0]:=PSECT(@FHeader._sectFat[0]);