Fixed truncation check, so 100 char names get GNU extension support when enabled

Fixed truncation check, so 100 char names get GNU extension support when enabled

Reported by Barrie Walker as follows:

I've experienced a problem with 1.2.11 where a name or link is exactly
100 characters.

When not using TAR_GNU name/linknames longer than 99 characters are
simply truncated to 99 characters (+1 for the NUL makes T_NAMELEN (=
100)). Any overflow is lost - fair enough.

Using TAR_GNU, name/linknames longer than 99 characters are still
truncated to 99 characters but the full string is placed in the
gnu_longname/gnu_longlink fields.
However the gnu_longname/gnu_longlink fields are populated only when a
name/linkname would be truncated.

The bug is that the check to see whether there will be truncation
compares the length against T_NAMELEN rather than T_NAMELEN-1.

The fix is simply, in lines 85 and 123 of encode.c, to change T_NAMELEN
to T_NAMELEN-1.