The prior logic would over-write the NUL-terminator if the octal value
was long enough. In order to prevent this, we add a fitsInOctal function
that does the proper check.

The relevant USTAR specification about NUL-terminator is:
<<<
Each numeric field is terminated by one or more <space> or NUL characters.
>>>

Change-Id: I6fbc6e8fe71168727eea201925d0fe08d43116ac
Reviewed-on: https://go-review.googlesource.com/54432Reviewed-by: Ian Lance Taylor <iant@golang.org>