Fix empty fields causing AR reader to fail

[rgmz]

This fixes #129.

[rgmz]

I added this struct because it makes errors clearer and debugging easier. If this isn't desirable, I can remove it.

[rgmz]

Hey @paultag, I was curious if you had any insights into this.

It seems that ar files can contain special entries (/ and // meaning symbol table and extended filename table respectively¹), hence why some fields are empty. My understanding is that the // entry and the /0 entry are for the same file. I'm not sure if my proposed fix is sufficient, or if additional pre-processing is desirable.

e.g., ar x long.a² results in a single file named name-longer-than-16-bytes.gox. This seems to differ from this library's current behaviour.

$ cat long.a
!<arch>
/               1478713642  0     0     0       4         `
//                                              32        `
name-longer-than-16-bytes.gox/

/0              1478713628  10556 5000  100664  577       `
ELF>�@@v1;
package foo;
pkgpath blah;
priority 1;
func F ();
checksum F9F6C8FE0C9E3CDD5A09D2996847B4E9992D8118;
.symtab.strtab.shstrtab.go_exporti�&0   @

Footnotes

1. https://en.wikipedia.org/wiki/Ar_(Unix)#System_V_(or_GNU)_variant ↩

2. https://github.com/openshift/ci-operator-prowgen/blob/ee31ec067b4670222de32ceecd01184b4eb52db3/vendor/golang.org/x/tools/go/gccgoexportdata/testdata/long.a ↩