avfilter/vf_separatefields: stop leaking last frame

This can happen when filtering is ended without receiving EOF.

libavfilter/vf_separatefields.c

@@ -118,6 +118,13 @@ static int request_frame(AVFilterLink *outlink)
     return ret;
 }
 
+static av_cold void uninit(AVFilterContext *ctx)
+{
+    SeparateFieldsContext *s = ctx->priv;
+
+    av_frame_free(&s->second);
+}
+
 static const AVFilterPad separatefields_inputs[] = {
     {
         .name         = "default",
@@ -141,6 +148,7 @@ AVFilter ff_vf_separatefields = {
     .name        = "separatefields",
     .description = NULL_IF_CONFIG_SMALL("Split input video frames into fields."),
     .priv_size   = sizeof(SeparateFieldsContext),
+    .uninit      = uninit,
     .inputs      = separatefields_inputs,
     .outputs     = separatefields_outputs,
 };