Sensors: sanitize baro before setting UAVO

Sometimes right at startup there seems to be an occasional
NAN in the altitude which can poison everything else.  I
considered fixing this in the MS5611 driver but it seems
reasonable to check before setting the UAVO regardless of
whether the MS5611 should have produced NAN.

I'm pushing this as a standalone PR instead of #672 because 
vinz saw some wobbles with that branch and this is the critical
fix.