Is 19V safe for charge Ni-Cd batteries?
19V would be too much for your NiCd. I don't recommend doing this. 

You can typically charge NiCd at about 1 C/hr; in your case, 2.2 A max. With 19V-6V overvoltage, you'll potentially fry the batteries pretty quickly. This is acutely dangerous. You don't want fire mixed with cadmium. Bad combination.

Get a charge controller. Or simply, a charger.