[#13,!7] Corrected a backwards check in billing class logic

RELNOTES - added a release note.

server/dhcp.c
    ack_lease () - corrected the logic used to capture the billing
    class name for logging

RELNOTES

@@ -129,6 +129,10 @@ by Eric Young (eay@cryptsoft.com).
   update ability (e.g. by undefining NSUPDATE).
   [ISC-Bugs, #16,!9 git ddb508ac083dae4ff83279dd240bad7f73a97b7d]
 
+- Corrected an issue that was causing the server, when running in
+  DHPCv4 mode, to segfault when class lease limits are reached.
+  [ISC-Bugs #13, !7  git #TBD]
+
 		Changes since 4.4.0 (New Features)
 - none
 		Changes since 4.4.0 (Bug Fixes)

server/dhcp.c

@@ -2562,7 +2562,7 @@ void ack_lease (packet, lease, offer, when, msg, ms_nulltp, hp)
 			int bill = 0;
 
 			for (i = 0; i < packet->class_count; i++) {
-				struct class *billclass, *subclass;
+				struct class *billclass, *superclass;
 
 				billclass = packet->classes[i];
 				if (billclass->lease_limit) {
@@ -2570,9 +2570,9 @@ void ack_lease (packet, lease, offer, when, msg, ms_nulltp, hp)
 					if (bill_class(lease, billclass))
 						break;
 
-					subclass = billclass->superclass;
-					if (subclass == NULL)
-						cname = subclass->name;
+					superclass = billclass->superclass;
+					if (superclass != NULL)
+						cname = superclass->name;
 					else
 						cname = billclass->name;
 				}