avfilter/vf_pullup: add comment to explain memset(0)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_pullup.c

@@ -137,7 +137,7 @@ static void free_field_queue(PullupField *head)
         av_free(f->combs);
         av_free(f->vars);
         next = f->next;
-        memset(f, 0, sizeof(*f));
+        memset(f, 0, sizeof(*f)); // clear all pointers to avoid stale ones
         av_free(f);
         f = next;
     } while (f != head);