FpSpreadsheet: Fix range overflow when a cell range is copied in spready to the clipboard.

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@9741 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/fpspreadsheet/source/common/xlsbiff8.pas

@@ -4539,6 +4539,12 @@ var
   len: Integer;
   wideStr: WideString;
 begin
+  if AString = '' then
+  begin
+    Result := 0;
+    exit;
+  end;
+
   // string constant is stored as widestring in BIFF8
   wideStr := UTF8Decode(AString);
   len := Length(wideStr);

components/fpspreadsheet/source/common/xlscommon.pas

@@ -5724,6 +5724,12 @@ var
   len: Byte;
   s: ansistring;
 begin
+  if AString = '' then
+  begin
+    Result := 0;
+    exit;
+  end;
+
   s := ConvertEncoding(AString, encodingUTF8, FCodePage);
   len := Length(s);
   AStream.WriteByte(len);