Add Device Wizard - Avoid crash on user nonsense

Trying to create a custom virtual power curve
with less than 2 points leads to a crash when
saving device config on terminate.
Reported at the users forum.

src/Train/AddDeviceWizard.cpp

@@ -1246,7 +1246,7 @@ void
 AddVirtualPower::myCreateCustomPowerCurve() {
 
     bool state = this->blockSignals(true);
-    if (state) return;
+    if (state || virtualPowerTableWidget->rowCount() < 2) return;
 
     T_MultiRegressionizer<XYVector<double>> fit(fitEpsilonSpinBox->value(), fitOrderSpinBox->value());