nbd: Don't crash when server reports NBD_CMD_READ failure
If a server fails a read, for example with EIO, but the connection
is still live, then we would crash trying to print a non-existent
error message in nbd_client_co_preadv(). For consistency, also
change the error printout in nbd_read_reply_entry(), although that
instance does not crash. Bug introduced in commit
f140e300.
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <
20171112013936.5942-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>