Serialize value: Fix check for simple value

Only THINGSET_ERR_UNSUPPORTED_FORMAT indicates that a value was not
a simple value. There may also be other error codes, which should
be propagated correctly.

Author: martinjaeger

src/thingset_bin.c

@@ -208,8 +208,8 @@ static int bin_serialize_value(struct thingset_context *ts,
     int err;
 
     err = bin_serialize_simple_value(ts->encoder, object->data, object->type, object->detail);
-    if (err == 0) {
-        return 0;
+    if (err != -THINGSET_ERR_UNSUPPORTED_FORMAT) {
+        return err;
     }
 
     /* not a simple value */

src/thingset_txt.c

@@ -247,7 +247,7 @@ static int txt_serialize_value(struct thingset_context *ts,
 
     int pos = json_serialize_simple_value(buf, size, object->data, object->type, object->detail);
 
-    if (pos < 0) {
+    if (pos == -THINGSET_ERR_UNSUPPORTED_FORMAT) {
         /* not a simple value */
         if (object->type == THINGSET_TYPE_GROUP) {
             pos = snprintf(buf, size, "null,");