Unconditionally call sftp_end() when input thread exits.

Hopefully fixes issue where certificate file is left open and so
can't be deleted.

src/sbbs3/main.cpp

@@ -2338,6 +2338,7 @@ void input_thread(void *arg)
 //		if(wr>100)
 //			mswait(500);	// Throttle sender
 	}
+	sbbs->sftp_end();
 	sbbs->online=false;
 	sbbs->sys_status|=SS_ABORT;	/* as though Ctrl-C were hit */
 	SetEvent(sbbs->inbuf.data_event); // terminate incom() wait

src/sbbs3/sftp.cpp

@@ -2164,7 +2164,8 @@ sbbs_t::init_sftp(int cid)
 bool
 sbbs_t::sftp_end(void)
 {
-	sftps_end(sftp_state);
+	if (sftp_state)
+		sftps_end(sftp_state);
 	sftp_state = nullptr;
 	return true;
 }