sys/syz-extract: provide readable error on missing input file
sys/syz-extract: provide readable error on missing input file

Currently syz-extract crashes with nil deref panic if
it is executed manually and is gives an absent file name.
Output a readable error.

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