Previous code seemed to assume that printf("%s", NULL) would result
in a 0-length string, not "(null)" or similar, but since there's no
point looking for files in "(null)/filepath...", instead we just
skip over NULL entries in search paths when generating file names.

In the *DirName() functions, this effectively just moves the "bail on
NULL in arg[i]" check up from the later code that assigned it to targetdir
and then bailed if that was NULL.

Not sure how there ever could be a NULL in arg[i], given the current
implementation of XlcParsePath, but it's easy enough to check once and
reject up front instead of on every reference.
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>