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.
diff --git a/include/proto/listener.h b/include/proto/listener.h
index e8f9ece..ca7dc9c 100644
--- a/include/proto/listener.h
+++ b/include/proto/listener.h
@@ -176,7 +176,7 @@
 	};
 	unsigned int st = l->state;
 
-	if (st > sizeof(states) / sizeof(*states))
+	if (st >= sizeof(states) / sizeof(*states))
 		return "INVALID";
 	return states[st];
 }