files: do not silently ignore an invalid backup directory, but die

It would be horrible if the user expects to find numbered backups
of all the files that were changed but they are NOT there because
the config file contains a typo or the relevant directory was moved
or renamed or something.  So... if the specified backup directory
is not usable, nano should complain and simply not start up.

src/files.c

@@ -1487,23 +1487,18 @@ int prompt_failed_backupwrite(const char *filename)
     return response;
 }
 
-/* Transform the specified backup directory to an absolute path. */
+/* Transform the specified backup directory to an absolute path,
+ * and verify that it is usable. */
 void init_backup_dir(void)
 {
-    char *full_backup_dir = get_full_path(backup_dir);
-
-    /* When we can't get an absolute path, or it's not a directory,
-     * cancel the making of backups. */
-    if (full_backup_dir == NULL ||
-		full_backup_dir[strlen(full_backup_dir) - 1] != '/') {
-	free(full_backup_dir);
-	free(backup_dir);
-	backup_dir = NULL;
-    } else {
-	free(backup_dir);
-	backup_dir = full_backup_dir;
-	snuggly_fit(&backup_dir);
-    }
+    backup_dir = free_and_assign(backup_dir, get_full_path(backup_dir));
+
+    /* If we can't get an absolute path (which means it doesn't exist or
+       isn't accessible), or it's not a directory, fail. */
+    if (backup_dir == NULL || backup_dir[strlen(backup_dir) - 1] != '/')
+	die(_("Invalid backup directory\n"));
+
+    snuggly_fit(&backup_dir);
 }
 #endif /* !NANO_TINY */
 

src/nano.c

@@ -2375,10 +2375,10 @@ int main(int argc, char **argv)
 #endif /* !DISABLE_HISTORIES */
 
 #ifndef NANO_TINY
-    /* If a backup directory was specified and we're not in restricted mode,
-     * make sure the path exists and is a directory, so that backup files can
-     * be saved there. */
-    if (backup_dir != NULL && !ISSET(RESTRICTED))
+    /* If backups are enabled and a backup directory was specified and
+     * we're not in restricted mode, make sure the path exists and is
+     * a directory, so that backup files can be saved there. */
+    if (ISSET(BACKUP_FILE) && backup_dir != NULL && !ISSET(RESTRICTED))
 	init_backup_dir();
 #endif