[perl #128740] Check for null in pp_ghostent et al.

[perl #128740] Check for null in pp_ghostent et al.

Specifically in the S_space_join_names_mortal static function that
several pp functions call.  On some platforms (such as Gentoo Linux
with torsocks), hent->h_aliases (where hent is a struct hostent *) may
be null after a gethostent call.