files: disallow tabbing when in restricted mode

Tabbing can list all files in the given directory, but restricted mode
is supposed to prevent the user from any access to the filesystem...

This fixes https://savannah.gnu.org/bugs/?58632.

Bug existed (in this form) since version 1.3.4, commit f7b5d930.

src/files.c

@@ -2076,8 +2076,8 @@ int do_writeout(bool exiting, bool withprompt)
 			answer = mallocstrcpy(answer, openfile->filename);
 		else {
 			/* Ask for (confirmation of) the filename.  Disable tab completion
-			 * when using restricted mode and the filename isn't blank. */
-			response = do_prompt(!ISSET(RESTRICTED) || openfile->filename[0] == '\0',
+			 * when in restricted mode. */
+			response = do_prompt(!ISSET(RESTRICTED),
 						MWRITEFILE, given, NULL,
 						edit_refresh, "%s%s%s", msg,
 #ifndef NANO_TINY