qt: Fix use of dangling pointer in QApplication

* qt/main.cpp (main): Use a new variable for argc that stays
valid.

The QApplication constructor takes argc as a reference and the referenced
integer has to stay alive for at least as long as the QApplication.

Based on a Patch from: Fabian Vogt <fvogt@suse.com>
Thanks!

• GnuPG-Bug-Id: T4658