Replace add(foo, NULL) with mh_xstrdup(foo).

Replace add(foo, NULL) with mh_xstrdup(foo).

add()'s arguments are back to front so add(foo, bar) produces bar+foo in
the normal case.  Thus add(foo, NULL) is read as the jarring NULL+foo.
Removing the NULL with mh_xstrdup() avoids this.  FENDNULL is used when
it isn't obvious foo can't be NULL as add() treats it as "" in that
case.