A comment in DropRole() contradicts the actual behavior

Hello,

Please look at errors, which produced by the following script, starting
from 6566133c5:
for i in `seq 100`; do (echo "CREATE USER u; DROP USER u;"); done | psql >psql-1.log 2>&1 &
for i in `seq 100`; do (echo "CREATE USER u; DROP USER u;"); done | psql >psql-2.log 2>&1 &
wait

ERROR:  could not find tuple for role 16387
ERROR:  could not find tuple for role 16390
ERROR:  could not find tuple for role 16394
...

Maybe these errors are expected, but then I'm confused by the comment in
DropRole():
         /*
          * Re-find the pg_authid tuple.
          *
          * Since we've taken a lock on the role OID, it shouldn't be possible
          * for the tuple to have been deleted -- or for that matter updated --
          * unless the user is manually modifying the system catalogs.
          */
         tuple = SearchSysCache1(AUTHOID, ObjectIdGetDatum(roleid));
         if (!HeapTupleIsValid(tuple))
             elog(ERROR, "could not find tuple for role %u", roleid);

Best regards,
Alexander