Fixed panic: runtime error: slice bounds out of range

[Apehaenger]

Hi Ned!

I use your ldap fork and got a:
panic: runtime error: slice bounds out of range ...
during some Search() request.

Here's the backtrace:

github.com/nmcclain/asn1-ber.decodePacket(0xc00068608b, 0x2, 0x8, 0xc000680a20, 0xc00068608b, 0x2, 0x8)
#011/home/jebeling/go/pkg/mod/github.com/nmcclain/asn1-ber@v0.0.0-20170104154839-2661553a0484/ber.go:335 +0x7e5
github.com/nmcclain/asn1-ber.decodePacket(0xc000686037, 0x5c, 0x5c, 0xc000680780, 0xc000686037, 0x5c, 0x5c)
#011/home/jebeling/go/pkg/mod/github.com/nmcclain/asn1-ber@v0.0.0-20170104154839-2661553a0484/ber.go:341 +0x279
github.com/nmcclain/asn1-ber.decodePacket(0xc000686006, 0x8d, 0x8d, 0xc0006804e0, 0xc000686006, 0x8d, 0x8d)
#011/home/jebeling/go/pkg/mod/github.com/nmcclain/asn1-ber@v0.0.0-20170104154839-2661553a0484/ber.go:341 +0x279
github.com/nmcclain/asn1-ber.decodePacket(0xc000686000, 0x93, 0x93, 0x90, 0x90, 0x0, 0x0)
#011/home/jebeling/go/pkg/mod/github.com/nmcclain/asn1-ber@v0.0.0-20170104154839-2661553a0484/ber.go:341 +0x279
github.com/nmcclain/asn1-ber.DecodePacket(...)
#011/home/jebeling/go/pkg/mod/github.com/nmcclain/asn1-ber@v0.0.0-20170104154839-2661553a0484/ber.go:304
github.com/nmcclain/asn1-ber.ReadPacket(0xe34c00, 0xc000278000, 0xc000278000, 0xe34c00, 0xc000278000)
#011/home/jebeling/go/pkg/mod/github.com/nmcclain/asn1-ber@v0.0.0-20170104154839-2661553a0484/ber.go:253 +0x56c
github.com/nmcclain/ldap.(*Server).handleConnection(0xc000245030, 0xe48de0, 0xc000278000)
#011/usr/src/Apehaenger/ldap/server.go:231 +0x2e5
created by github.com/nmcclain/ldap.(*Server).Serve

Most (but not all) of the code got copied from Brian Ollenberger's 'Add bounds checks to prevent slice out of range panics' patch from commit https://github.com/bollenberger/asn1-ber/commit/4f968084f91b2896cfa8607310c926773b88e9e5

In addition I slightly aligned the debug output and added a go.mod file to be able to use go's replace for a local dev version.