Drop closeTxd on a discarded close

SendChannelClose() latches closeTxd on the same terms as the EOF beside it: on
anything but the send failure that discards the output buffer. It latched
unconditionally, so a closeTxd claiming a close that never left made
wolfSSH_shutdown()'s gate skip the teardown altogether.

- Split from the EOF latch on purpose: that one widens, from success-only to
  bundled, and this one narrows. Read as one change they read wrong.

src/internal.c

@@ -20492,7 +20492,10 @@ int SendChannelClose(WOLFSSH* ssh, word32 peerChannelId)
 
     if (ret == WS_SUCCESS) {
         ret = wolfSSH_SendPacket(ssh);
-        channel->closeTxd = 1;
+        /* Same terms as SendChannelEof(). A closeTxd for a close that never
+         * left makes wolfSSH_shutdown() skip the teardown. */
+        if (ret != WS_SOCKET_ERROR_E || wolfSSH_OutputPending(ssh))
+            channel->closeTxd = 1;
     }
 
     WLOG(WS_LOG_DEBUG, "Leaving SendChannelClose(), ret = %d", ret);

tests/unit.c

@@ -7327,6 +7327,12 @@ static int test_SendChannelEofSendFails(void)
     if (ch->eofTxd) { result = -1692; goto done; }
     if (ssh->outputBuffer.length != 0) { result = -1693; goto done; }
 
+    /* SendChannelClose() has the same shape, and a closeTxd claiming a close
+     * that never left makes wolfSSH_shutdown() skip the teardown. */
+    ret = SendChannelClose(ssh, ch->peerChannel);
+    if (ret != WS_SOCKET_ERROR_E) { result = -1762; goto done; }
+    if (ch->closeTxd) { result = -1763; goto done; }
+
     /* So the channel is not half-closed, and an EOF can still be built and
      * sent once the socket takes bytes again. */
     wolfSSH_SetIOSend(ctx, DiscardIoSend);