BUG/MINOR: listener: fix off-by-one in state name check

As reported in issue #380, the state check in listener_state_str() is
invalid as it allows state value 9 to report crap. We don't use such
a state value so the issue should never happen unless the memory is
already corrupted, but better clean this now while it's harmless.

This should be backported to all maintained branches.
1 file changed