Fix bug 8040 - smbclient segfaults when a Cyrillic netbios name or workgroup is confi...

Fix bug 8040 - smbclient segfaults when a Cyrillic netbios name or workgroup is configured.

As discovered by David Disseldorp <ddiss@suse.de>, convert_string_talloc()
doesn't always return consistent results for a zero length string. The
API states an incoming string must *always* contain the terminating null,
but unfotunately too much code expects passing in a zero source length
to return a null terminated string, so at least ensure we return a
correct null string in the required character set and return the
correct length.

Also ensure we cannot return a zero length for a converted string
(we ensure that the returned buffer is always allocated and zero
terminated anyway) as calling code depends on the fact that returning
true from this function will *always* return a non-zero length (as
it must include the terminating null).

Note this is a different fix from what went into master (this is
identical to the fix I'm planning for 3.5.x) as convert_string_talloc()
has diverged between the two.

Jeremy.