AVOptions: set string default values.

libavutil/opt.c

@@ -443,8 +443,10 @@ void av_opt_set_defaults2(void *s, int mask, int flags)
             }
             break;
             case FF_OPT_TYPE_STRING:
+                av_set_string3(s, opt->name, opt->default_val.str, 1, NULL);
+                break;
             case FF_OPT_TYPE_BINARY:
-                /* Cannot set default for string as default_val is of type * double */
+                /* Cannot set default for binary */
             break;
             default:
                 av_log(s, AV_LOG_DEBUG, "AVOption type %d of option %s not implemented yet\n", opt->type, opt->name);