Erlang: Revert breaking change

Revert 6542a36
xavp_add_xavp_value returns the root and not the xavp it added and that was the reason for the code the commit removed.  Changed a variable name to help make that more clear.

Also related to #2069.

(cherry picked from commit 1ed58f0)

src/modules/erlang/pv_xbuff.c

@@ -86,11 +86,15 @@ sr_xavp_t *xbuff_new(str *name)
 
 	if(!xbuffs_root)
 	{
-		xbuff = xavp_add_xavp_value(&xbuff_list,name,&xbuff_val,xavp_get_crt_list());
-	} else {
-		xbuff = xavp_get_child(&xbuff_list, name);
+		xbuffs_root = xavp_add_xavp_value(&xbuff_list,name,&xbuff_val,xavp_get_crt_list());
+		if (!xbuffs_root){
+				LM_ERR("cannot create xbuffs_root \n");
+				return NULL;
+			}
 	}
 
+	xbuff = xavp_get_child(&xbuff_list, name);
+
 	if (!xbuff) {
 
 		xbuff_val.type = SR_XTYPE_NULL;