qt: Fix crash when setting repeat error

* qt/pinentrydialog.cpp (PinEntryDialog::setRepeatErrorText): Check
for nullptr.

This fixes a crash if pinentry is used for something other than asking
for a new password.

• GnuPG-bug-id: T5543
• Fixes-commit: 8acf47ea46a6c2aa1fc5254b81c6cf57ad506b9c