gc: fix cast in compare_tasks_by_selection()

compare_tasks_by_selection() is used with QSORT and gets passed pointers
to the elements of "static struct maintenance_task tasks[]".  It casts
the *addresses* of these passed pointers to element pointers, though,
and thus effectively compares some unrelated values from the stack.  Fix
the casts to actually compare array elements.

Detected by USan (make SANITIZE=undefined test).

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

builtin/gc.c

@@ -1253,10 +1253,8 @@ static struct maintenance_task tasks[] = {
 
 static int compare_tasks_by_selection(const void *a_, const void *b_)
 {
-	const struct maintenance_task *a, *b;
-
-	a = (const struct maintenance_task *)&a_;
-	b = (const struct maintenance_task *)&b_;
+	const struct maintenance_task *a = a_;
+	const struct maintenance_task *b = b_;
 
 	return b->selected_order - a->selected_order;
 }