avfilter/vf_framerate: fix frame leak at uninit

Reported-by: Andres Noetzli
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/vf_framerate.c

@@ -526,7 +526,7 @@ static av_cold void uninit(AVFilterContext *ctx)
     FrameRateContext *s = ctx->priv;
     int i;
 
-    for (i = s->frst + 1; i > s->last; i++) {
+    for (i = s->frst + 1; i < s->last; i++) {
         if (s->srce[i] && (s->srce[i] != s->srce[i + 1]))
             av_frame_free(&s->srce[i]);
     }