now_seg after closing output file

now_seg, a pointer into the output file sections, isn't valid after
the output file is closed.  gas doesn't and shouldn't use now_seg
after this point of course, but let's be safe.

	* output-file.c (output_file_close): Clear now_seg and now_subseg.

gas/output-file.c

@@ -99,6 +99,8 @@ output_file_close (void)
     res = bfd_cache_close_all ();
   else
     res = bfd_close (obfd);
+  now_seg = NULL;
+  now_subseg = 0;
 
   filename = out_file_name;
   out_file_name = NULL;