eval: fix memleak.

(cherry picked from commit fe277b16f0)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/eval.c

@@ -488,6 +488,7 @@ int av_expr_parse(AVExpr **expr, const char *s,
     if ((ret = parse_expr(&e, &p)) < 0)
         goto end;
     if (*p.s) {
+        av_expr_free(e);
         av_log(&p, AV_LOG_ERROR, "Invalid chars '%s' at the end of expression '%s'\n", p.s, s0);
         ret = AVERROR(EINVAL);
         goto end;