transfused perfstat: fix bug with running multiple perfstats in a row (#1129)

After freeing the head of the perfstat block list, we must set it to NULL
to indicate that new blocks must be allocated. Previously, we risked
segfaults by trying to use a freed list.

Signed-off-by: David Sheets <dsheets@docker.com>

alpine/packages/transfused/transfused_perfstat.c

@@ -165,6 +165,7 @@ void *stop_perfstat(parameters_t *params, char *req, size_t len)
 	*((uint64_t *) (reply + 8)) = now(params);
 
 	copy_and_free_perfstats(conn->perfstats, reply + 16);
+	conn->perfstats = NULL;
 
 	unlock("perfstat lock: stop_perfstat", &conn->perfstat_lock);