pkg/cover: fix coverage PC filter
pkg/cover: fix coverage PC filter

The result is not updated because it modifies the deep copy variable prog.
Fix this by using array indexing.

google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller