Fix for 0.18.1

[genotrance]

Using #head, needed this change along with minor changes to readStr() and peekStr() to work around ""[0] issue.

Tested with 0.18.0 as well as #head.

[dom96]

What problem does this solve? Ignoring errors like this usually leads to trouble.

[genotrance]

When the filename field is blank - 101 blank characters, the filesize field = 0000000000000000000000 which fails the parseOctInt() function. This just catches that error.

I don't know why 0.18.0 handles this though.

[genotrance]

Reason it fails is due to this commit:-

nim-lang/Nim@9b8603a#diff-d48a4a282bdbb4254c40b51e199e0a2cL1763

of '\0': break is no longer present and since header[124 .. 134] filesize is all nulled out for a blank filename, parseOctInt() is raising a ValueError.

[data-man]

I haven't checked in detail what kind of tar supports this library. And how packed a files.
Maybe this will help:
https://github.com/libarchive/libarchive/blob/master/libarchive/archive_read_support_format_tar.c#L54

[dom96]

So maybe the stdlib should be fixed, not untar?

[genotrance]

@Araq explicitly removed null checks a month ago, I'll let him decide.

[data-man]

@genotrance

Check with this, please: https://gist.github.com/data-man/f750df0593c71daacca1e5cb5c5b5573