CLEANUP: opentracing: added variable to store variable length

The same variable should not be used to store multiple different results,
because it can be confusing.  Therefore, the var_name_len variable has
been added in several functions, in order to avoid the use of the retval
variable for several different purposes.

This patch must be backported as far as 2.4.
diff --git a/addons/ot/src/vars.c b/addons/ot/src/vars.c
index 3d02c68..daaa149 100644
--- a/addons/ot/src/vars.c
+++ b/addons/ot/src/vars.c
@@ -280,26 +280,26 @@
 {
 	struct arg arg;
 	char       var_name[BUFSIZ];
-	int        retval;
+	int        retval = -1, var_name_len;
 
 	FLT_OT_FUNC("\"%s\", \"%s\", \"%s\", %p:%p", scope, prefix, name, FLT_OT_DPTR_ARGS(err));
 
-	retval = flt_ot_var_name(scope, prefix, name, var_name, sizeof(var_name), err);
-	if (retval == -1)
+	var_name_len = flt_ot_var_name(scope, prefix, name, var_name, sizeof(var_name), err);
+	if (var_name_len == -1)
 		FLT_OT_RETURN(retval);
 
 	/* Set <size> to 0 to not release var_name memory in vars_check_arg(). */
 	(void)memset(&arg, 0, sizeof(arg));
 	arg.type          = ARGT_STR;
 	arg.data.str.area = var_name;
-	arg.data.str.data = retval;
+	arg.data.str.data = var_name_len;
 
 	if (vars_check_arg(&arg, err) == 0) {
 		FLT_OT_ERR_APPEND("failed to register variable '%s': %s", var_name, *err);
-
-		retval = -1;
 	} else {
 		FLT_OT_DBG(2, "variable '%s' registered", arg.data.var.name);
+
+		retval = var_name_len;
 	}
 
 	FLT_OT_RETURN(retval);
@@ -329,22 +329,22 @@
 {
 	struct sample smp;
 	char          var_name[BUFSIZ];
-	int           retval;
+	int           retval = -1, var_name_len;
 
 	FLT_OT_FUNC("%p, \"%s\", \"%s\", \"%s\", \"%s\", %u, %p:%p", s, scope, prefix, name, value, opt, FLT_OT_DPTR_ARGS(err));
 
-	retval = flt_ot_var_name(scope, prefix, name, var_name, sizeof(var_name), err);
-	if (retval == -1)
+	var_name_len = flt_ot_var_name(scope, prefix, name, var_name, sizeof(var_name), err);
+	if (var_name_len == -1)
 		FLT_OT_RETURN(retval);
 
 	flt_ot_smp_init(s, &smp, opt, SMP_T_STR, value);
 
-	if (vars_set_by_name_ifexist(var_name, retval, &smp) == 0) {
+	if (vars_set_by_name_ifexist(var_name, var_name_len, &smp) == 0) {
 		FLT_OT_ERR("failed to set variable '%s'", var_name);
-
-		retval = -1;
 	} else {
 		FLT_OT_DBG(2, "variable '%s' set", var_name);
+
+		retval = var_name_len;
 	}
 
 	FLT_OT_RETURN(retval);