MINOR: server: irrelevant error message with 'default-server' config file keyword.

There is no reason to emit such an error message:
"'default-server' expects <name> and <addr>[:<port>] as arguments."
if less than two arguments are provided on 'default-server' lines.
This is a 'server' specific error message.
diff --git a/src/server.c b/src/server.c
index 5589723..563d38d 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1020,7 +1020,7 @@
 		else if (warnifnotcap(curproxy, PR_CAP_BE, file, linenum, args[0], NULL))
 			err_code |= ERR_ALERT | ERR_FATAL;
 
-		if (!*args[2]) {
+		if (!defsrv && !*args[2]) {
 			Alert("parsing [%s:%d] : '%s' expects <name> and <addr>[:<port>] as arguments.\n",
 			      file, linenum, args[0]);
 			err_code |= ERR_ALERT | ERR_FATAL;