Avoid possible use of uninitialized variable

Errorcode doesn't get set if duppedfd is negative. So let's just issue an
error and bail.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/subsurfacewebservices.cpp

@@ -813,6 +813,10 @@ void DivelogsDeWebServices::downloadFinished()
 		zip = zip_fdopen(duppedfd, 0, &errorcode);
 		if (!zip)
 			::close(duppedfd);
+	} else {
+		QMessageBox::critical(this, tr("Problem with download"),
+				      tr("The archive could not be opened:\n"));
+		return;
 	}
 #else
 	struct zip *zip = zip_open(QFile::encodeName(zipFile.fileName()), 0, &errorcode);