One of the most important things we're taught in school is to NEVER IGNORE A FIRE ALARM.  But on the "NBC Nightly News" last night, that's exactly what BRIAN WILLIAMS did.       The fire alarm went off at the beginning of the broadcast, and even though it was loud and annoying, Brian only stopped to tell viewers what all the noise was, and to let them know that he wasn't in any danger. Then he continued with the show.