Why TF would anyone write software that decided that the best thing to do with an impossible command was to stall the aircraft?? - I'm sure I'd have been sacked if I'd ever done anything like that.
Autopilots on large transport a/c like the B737 have reversion modes - if you climb in VS and the speed drops to a certain figure the mode will change so that you do not stall (although the resultant speed is below the normal minimum).
Climbing in VS with an autopilot with no reversion mode is a potential threat especially at higher levels when the performance is running out of steam.