tweaks: avoid passing NULL to access()

This prevents an unwanted message when nano
is compiled with -fsanitize=undefined.

This fixes https://savannah.gnu.org/bugs/?63447.

Problem existed since version 4.8, commit 343f97b3,
since the --rcfile option was added.

src/rcfile.c

@@ -1707,7 +1707,7 @@ void do_rcfiles(void)
 {
 	if (custom_nanorc) {
 		nanorc = get_full_path(custom_nanorc);
-		if (access(nanorc, F_OK) != 0)
+		if (nanorc == NULL || access(nanorc, F_OK) != 0)
 			die(_("Specified rcfile does not exist\n"));
 	} else
 		nanorc = mallocstrcpy(nanorc, SYSCONFDIR "/nanorc");