Fix off by one error in wxFTP::GetFileSize().

Fix off by one error in wxFTP::GetFileSize().

We incremented the index once more even after finding the line we were looking
for in the array which meant that we accessed a wrong array element in any
case and could even attempt to access an out of bound one if the file was
found in the last line.

Closes #11964.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64105 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775