galera: Ignore safe_to_bootstrap in grastate.dat in some cases

We've seen a case where one node in the cluster had safe_to_bootstrap as
1 in grastate.dat, but apparently the UUID and the last commit (or
seqno) were 00000000-0000-0000-0000-000000000000 and -1 -- which matches
a fresh state.

In that case, we do not want to use that node as a bootstrap node, as it
causes a risk of losing everything.