You'd have to brake REALLY hard to completely stop the front wheel turning though! Try sometime when you're on a carpark or somewhere away from traffic - brake harder and harder, throwing your weight back and bracing with your arms until you are lifting the rear wheel. Don't brake so hard you go over, just so the rear lifts a few cm.

If you watch, the front hasn't stopped completely. It is just turning too slowly for the deceleration of the bike. With practice this is easy and it is a good skill to have - lets you know what your maximum safe braking is.

In any case, it hasn't "locked up" as in stoppped. That might happen on loose terrain but on good roads and grippy tyres I don't think it is possible.