signature parse: check for strtol error through errno

[carlosmn]

strtol can return 0 on success. Check if errno is set to determine
object corruption.

Signed-off-by: Carlos Martín Nieto cmn@elego.de

[carlosmn]

If we know that there is no space before the timestamp, we could also check to see if the buffer has moved, but in this case I think we in fact know there is at least one space.

[vmg]

Hey, thanks for spotting this, but the fix is not valid: the POSIX standard doesn't require errno to be set when strtol fails. I've fixed this by adding a custom strol implementation that does proper error checking. Cheers!