MINOR: standard: make memprintf() support a NULL destination

Doing so removes many checks that were systematically made because
the callees don't know if the caller passed a valid pointer.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index 3757236..3f18770 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -352,8 +352,7 @@
 static int bind_parse_mode(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
 {
 	if (!*args[cur_arg + 1]) {
-		if (err)
-			memprintf(err, "'%s' : missing mode (octal integer expected)", args[cur_arg]);
+		memprintf(err, "'%s' : missing mode (octal integer expected)", args[cur_arg]);
 		return ERR_ALERT | ERR_FATAL;
 	}
 
@@ -365,8 +364,7 @@
 static int bind_parse_gid(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
 {
 	if (!*args[cur_arg + 1]) {
-		if (err)
-			memprintf(err, "'%s' : missing value", args[cur_arg]);
+		memprintf(err, "'%s' : missing value", args[cur_arg]);
 		return ERR_ALERT | ERR_FATAL;
 	}
 
@@ -380,15 +378,13 @@
 	struct group *group;
 
 	if (!*args[cur_arg + 1]) {
-		if (err)
-			memprintf(err, "'%s' : missing group name", args[cur_arg]);
+		memprintf(err, "'%s' : missing group name", args[cur_arg]);
 		return ERR_ALERT | ERR_FATAL;
 	}
 
 	group = getgrnam(args[cur_arg + 1]);
 	if (!group) {
-		if (err)
-			memprintf(err, "'%s' : unknown group name '%s'", args[cur_arg], args[cur_arg + 1]);
+		memprintf(err, "'%s' : unknown group name '%s'", args[cur_arg], args[cur_arg + 1]);
 		return ERR_ALERT | ERR_FATAL;
 	}
 
@@ -400,8 +396,7 @@
 static int bind_parse_uid(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
 {
 	if (!*args[cur_arg + 1]) {
-		if (err)
-			memprintf(err, "'%s' : missing value", args[cur_arg]);
+		memprintf(err, "'%s' : missing value", args[cur_arg]);
 		return ERR_ALERT | ERR_FATAL;
 	}
 
@@ -415,15 +410,13 @@
 	struct passwd *user;
 
 	if (!*args[cur_arg + 1]) {
-		if (err)
-			memprintf(err, "'%s' : missing user name", args[cur_arg]);
+		memprintf(err, "'%s' : missing user name", args[cur_arg]);
 		return ERR_ALERT | ERR_FATAL;
 	}
 
 	user = getpwnam(args[cur_arg + 1]);
 	if (!user) {
-		if (err)
-			memprintf(err, "'%s' : unknown user name '%s'", args[cur_arg], args[cur_arg + 1]);
+		memprintf(err, "'%s' : unknown user name '%s'", args[cur_arg], args[cur_arg + 1]);
 		return ERR_ALERT | ERR_FATAL;
 	}