Use validated index in json_type_to_name.

Another currently unreachable issue. The enum o_type was casted to an
int and this int has been validated to be an index.

Therefore use the int, not the enum as index. Defensive measure for
a probably never occurring future in which the enum would contain
values larger than INT_MAX.

json_util.c

@@ -292,5 +292,5 @@ const char *json_type_to_name(enum json_type o_type)
 		                     NELEM(json_type_name));
 		return NULL;
 	}
-	return json_type_name[o_type];
+	return json_type_name[o_type_int];
 }