Bugfix: Check that QRcode_encodeString didn't return NULL (error)

Without this, any error will segfault Bitcoin-Qt

src/qt/qrcodedialog.cpp

@@ -41,6 +41,11 @@ void QRCodeDialog::genCode()
         ui->lblQRCode->setText("");
 
         QRcode *code = QRcode_encodeString(uri.toUtf8().constData(), 0, QR_ECLEVEL_L, QR_MODE_8, 1);
+        if (!code)
+        {
+            ui->lblQRCode->setText(tr("Error encoding URI into QR Code."));
+            return;
+        }
         myImage = QImage(code->width + 8, code->width + 8, QImage::Format_RGB32);
         myImage.fill(0xffffff);
         unsigned char *p = code->data;