lavfi/tinterlace: set inlink->cur to NULL, since it is stored internally

If not set to NULL, the reference is freed by ff_end_frame(), and later
accessed in end_frame() by the filter code, causing a crash.

libavfilter/vf_tinterlace.c

@@ -206,6 +206,7 @@ static int start_frame(AVFilterLink *inlink, AVFilterBufferRef *picref)
     avfilter_unref_buffer(tinterlace->cur);
     tinterlace->cur  = tinterlace->next;
     tinterlace->next = picref;
+    inlink->cur_buf = NULL;
     return 0;
 }