Check that target is string and value is not NULL in json_string_set()

src/value.c

@@ -703,6 +703,9 @@ int json_string_set_nocheck(json_t *json, const char *value)
     char *dup;
     json_string_t *string;
 
+    if(!json_is_string(json) || !value)
+        return -1;
+
     dup = jsonp_strdup(value);
     if(!dup)
         return -1;