Allow fill_archive to be called with NULL fname.

The fill_archive function should support being
called with a NULL fname argument. A NULL fname
argument indicates that the default archive should
be opened. We enable this by setting ah.fname to
NULL before initializing ah or calling open_archive.
This way if fname is NULL then ah.fname remains NULL
and open_archive opens the default archive.

build-locale-archive.c

@@ -311,6 +311,7 @@ fill_archive (struct locarhandle *tmpl_ah,
 
   /* Open the archive.  This call never returns if we cannot
      successfully open the archive.  */
+  ah.fname = NULL;
   if (fname != NULL)
     ah.fname = fname;
   open_archive (&ah, false);