prog: fix pointer validation
prog: fix pointer validation

Query size after validating the object itself,
otherwise size can panic on corrupted object.

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