fpspreadsheet: No "invalid index" error message when attempting to save a workbook without reader/writer unit in "uses"

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@7197 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/fpspreadsheet/source/common/fpsreaderwriter.pas

@@ -1199,6 +1199,7 @@ begin
   begin
     // Bring the priority format to the top
     idx := IndexOf(APriorityFormat);
+    if idx > -1 then
       FList.Exchange(0, idx);
   end;
 
@@ -1224,6 +1225,7 @@ begin
 
   if APriorityFormat <> sfidUnknown then
     // Restore original order
+    if idx > -1 then
       FList.Exchange(idx, 0);
 end;