Improve check for overly-long extensible node name.
The old code is bad for two reasons. First, it has an off-by-one
error. Second, it won't help if you aren't running with assertions
enabled. Per discussion, we want a check here in that case too.
Author: KaiGai Kohei, adjusted by me.
Reviewed-by: Petr Jelinek
Discussion:
56E0D547.
1030101@2ndquadrant.com