Fix release of empty IPv6 active lease

RELNOTES

@@ -86,6 +86,9 @@ suggested fixes to <dhcp-users@isc.org>.
 - NoAddrsAvail answers to SOLICITs are always ADVERTISEs even when a SOLICIT
   carries a rapid-commit option.
 
+- Return in place of raise an impossible condition when one tries to release
+  an empty active lease.
+
 			Changes since 4.0.0b3
 
 - The reverse dns name for PTR updates on IPv6 addresses has been fixed to

client/dhc6.c

@@ -1247,7 +1247,7 @@ do_release6(void *input)
 	client = input;
 
 	if (client->active_lease == NULL)
-		log_fatal("Impossible condition at %s:%d.", MDL);
+		return;
 
 	if ((client->MRC != 0) && (client->txcount > client->MRC))  {
 		log_info("Max retransmission count exceeded.");