util: json: Improve handling and docs for adding JSON objects

The JSON structure constructor has an option to add JSON arrays to the
constructed object. The description is inaccurate as it can add any json
object even a dict. Change the docs to cover this option and reject
adding NULL objects.

src/util/virjson.c

@@ -97,7 +97,8 @@ struct _virJSONParser {
  *
  * d: double precision floating point number
  * n: json null value
- * a: json array
+ *
+ * a: json object, must be non-NULL
  *
  * The value corresponds to the selected type.
  *
@@ -231,6 +232,14 @@ virJSONValueObjectCreateVArgs(virJSONValuePtr *obj, va_list args)
 
         case 'a': {
             virJSONValuePtr val = va_arg(args, virJSONValuePtr);
+
+            if (!val) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("argument key '%s' must not have null value"),
+                               key);
+                goto cleanup;
+            }
+
             rc = virJSONValueObjectAppend(jargs, key, val);
         }   break;