Once ExportKeyStreamResource::handleRequest is entered, a 'Save As'
dialog will be opened by the browser, even if key data is empty. This
may happen if passphrase is wrong.
If key data is generated before the download is actually requested, we
can check if there is anything to push to browser and prevent the
download request.
Requests the passphrase with a popup.
As from GPGME 1.15.0, the loopback pinentry is functional when exporting secret keys. It works fine when the exact passphrase is provided. If it's a wrong passphrase, GPGME does not generate an ::Error, but the app crashes with 'free(): double free detected in tcache 2'.
Hence, this patch cannot be committed to master.
Status : dangerous
Result : works and works not
Reason : a wrong passphrase means a crash
