gram.y: ensure prefix in T_Deprecate* rule

If auto-section of prefixes "prefix ::/64" doesn't find any usable
prefixes, prefix will be null.  All the prefix options rules need
to check that prefix is non-null before attempting to assign values
to it.
process.c: make sure we wait for random delay

When receiving an RS, we need to wait a random amount of time before
sending out our RA to avoid packet collision in the lower layers of
the network stack.  As far as I know, the lower levels will deal with
the collision, but the RADVD RFC calls for a random delay so this
change is making making sure it happens.
Merge branch 'delay-time'
Update CHANGES with latest fixes