vf_frei0r: do not increment string if it reached the end

Bug-Id: 778
CC: libav-stable@libav.org

libavfilter/vf_frei0r.c

@@ -149,6 +149,7 @@ static int set_params(AVFilterContext *ctx, const char *params)
         if (*params) {
             if (!(param = av_get_token(&params, "|")))
                 return AVERROR(ENOMEM);
+            if (*params)
                 params++;               /* skip ':' */
             ret = set_param(ctx, info, i, param);
             av_free(param);