`-B` fails with `lseek: Invalid argument` if it would start before the beginning of the file

Would be better to just start from 0 instead in that case:

```
printf '\x01\x02\x03' > tmp
```

Good:

```
bgrep -B2 03 tmp
```

output:

```
tmp: 00000002
\x01\x02
```

Bad:

```
bgrep -B3 03 tmp
```

output:

```
tmp: 00000002
lseek: Invalid argument
```