executor: don't let syscalls fail with errno=0
executor: don't let syscalls fail with errno=0

Our syz syscalls may mishandle errno in some cases
and fail with errno=0. Fix it up.

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