Portable decoder doesn't return error for output buffer too short

[greatroar]

The following tries to decode into a buffer that is too short:

// 14 bytes literal, 3+minMatch=7 bytes match, for 21 bytes total.
src := []byte("\xe300000000000000\t\x00\x00")
dst := make([]byte, 18)
lz4.UncompressBlock(src, dst)

The asm decoders correctly detect that the output buffer is too short. The portable decoder returns 21, nil.

The problem is in "Shortcut 2" of the portable decoder. Commenting that out makes it work. It should probably require more space in dst, but that was removed as part of the fix for #51.

[greatroar]

In fact, just commenting out

if end > uint(len(dst)) {
    // The remaining buffer may not hold 18 bytes.
    // See https://github.com/pierrec/lz4/issues/51.
    end = uint(len(dst))
}

makes it work, without TestIssue51 failing.