Use full paths when checking for existence in wxGenericFileDialog

Use full path to the selected file before checking whether it exists,
otherwise the check could fail even if the file does exist but the current
directory is different from the one it is in.

Closes #16698.

(this is a backport of 5846144614 from
master)

src/generic/filedlgg.cpp

@@ -339,7 +339,7 @@ bool wxGenericFileDialog::Show( bool show )
 void wxGenericFileDialog::OnOk( wxCommandEvent &WXUNUSED(event) )
 {
     wxArrayString selectedFiles;
-    m_filectrl->GetFilenames(selectedFiles);
+    m_filectrl->GetPaths(selectedFiles);
 
     if (selectedFiles.Count() == 0)
         return;