ospf6d: Fix LSA formatting inconsistent retvals

ospf6d: Fix LSA formatting inconsistent retvals

Make return values for lh_get_prefix_str LSA handlers consistent, i.e.
return NULL in case of error without having written to the passed buffer
and non-NULL (address of buffer) if a string was written to the buffer.

Previously, it was possible in certain cases (bogus LSAs) to not initialize
(and 0-terminate) the buffer but still return non-NULL, causing the caller
to print random junk.

Signed-off-by: Martin Buck <mb-tmp-tvguho.pbz@gromit.dyndns.org>