startup: don't crash when dying early

That is: don't try to look at open files when none are open yet.

src/nano.c

@@ -605,7 +605,7 @@ void die(const char *msg, ...)
 
 #ifndef NANO_TINY
     /* If the current buffer has a lockfile, remove it. */
-    if (ISSET(LOCKING) && openfile->lock_filename)
+    if (openfile && ISSET(LOCKING) && openfile->lock_filename)
 	delete_lockfile(openfile->lock_filename);
 #endif