rowcount() outputs number of rows 0

```
What steps will reproduce the problem?
1. try to read large excel sheet that has 65536 rows.
$xls = new Spreadsheet_Excel_Reader("filename.xls");
echo $xls->rowcount();

What is the expected output? What do you see instead?

Expected : It should output the number of rows int the sheet.

Current : Output is 0.

Please provide any additional information below.

Note : It gives correct number of rows for a file with less number of rows.
(e.g: i tried with a sheet that had 500 rows)
output : 500

```

Original issue reported on code.google.com by `amber.ku...@gmail.com` on 8 Nov 2013 at 9:19

Start at Spreadsheet_Excel_Reader->rowcount() and reproduce the problem with an XLS sheet containing 65,536 rows, comparing it with the reported 500-row case. Done means rowcount() returns the sheet's actual row count for the large file while preserving the correct result for smaller files; no test file is mentioned.

Written by the indexing model from the issue text.