[MINOR] frontend: ensure debug message length is always initialized

If the socket family ever changes from AF_INET*/AF_UNIX, we'd have a problem.
diff --git a/src/frontend.c b/src/frontend.c
index a295729..4ed4862 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -178,7 +178,7 @@
 
 	if (unlikely((global.mode & MODE_DEBUG) && (!(global.mode & MODE_QUIET) || (global.mode & MODE_VERBOSE)))) {
 		char pn[INET6_ADDRSTRLEN];
-		int len;
+		int len = 0;
 
 		if (!(s->flags & SN_FRT_ADDR_SET))
 			get_frt_addr(s);