Don't rely on unspecified behavior

Calling access(p, m) with p == NULL is not specified, so don't do that.  On
GNU/Hurd systems doing so will result in a SIGSEGV.

Signed-off-by: Thomas Schwinge <tschwinge@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-add.c

@@ -60,7 +60,7 @@ static void fill_directory(struct dir_struct *dir, const char **pathspec,
 		path = git_path("info/exclude");
 		if (!access(path, R_OK))
 			add_excludes_from_file(dir, path);
-		if (!access(excludes_file, R_OK))
+		if (excludes_file != NULL && !access(excludes_file, R_OK))
 			add_excludes_from_file(dir, excludes_file);
 	}