Fix string iteration logic in wxFTP::Pwd()

wxString iterators can't be dereferenced once they reach the end of the
string, so compare them with end rather than checking if the value they point
to is non-NUL.

This makes wxFTP::Pwd() actually work, which was apparently broken since quite
some time (perhaps ever since c9f7896861 9+
years ago).

See #17679.

(cherry picked from commit d0c57dbef0)

docs/changes.txt

@@ -587,6 +587,7 @@ All:
 
 - Fix wxFileName::MakeRelativeTo() for directory relatively to itself.
 - Fix wxLocale::IsOk() return true even if setting the locale failed.
+- Fix wxFTP::Pwd() to actually work.
 - Null-terminate wxApp::argv for compatibility with the real argv.
 
 Unix: