Fixes memory leak when releasing cell's memory.

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@783 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/fpspreadsheet/fpspreadsheet.pas

@@ -217,6 +217,9 @@ end;
 }
 procedure TsWorksheet.RemoveCallback(data, arg: pointer);
 begin
+  { The UTF8STring must be manually reseted to nil content, because
+    FreeMem only frees the record mem, without checking its content }
+  PCell(data).UTF8StringValue:='';
   FreeMem(data);
 end;