fix: hanging process when TLS alert happens during mqtt_connect (tls_alert)

Similar to #279

This time, the error is a bit different, but the effect is the same:

```
18:17:09.437 [info] [reason: {:tls_alert, {:certificate_revoked, ~c"TLS client: In state connection received SERVER ALERT: Fatal - Certificate Revoked\n"}}, msg: ~c"failed_to_send_connect_packet", clientid: "emqtt-erlang-9e2056f5415da439bfce"]
```

```
Testing apps.emqx.emqx_crl_cache_SUITE: *** FAILED test case 1 ***
%%% emqx_crl_cache_SUITE ==> t_revoked: FAILED
%%% emqx_crl_cache_SUITE ==> {{case_clause,{error,connack_timeout}},
 [{emqx_crl_cache_SUITE,t_revoked,1,
                        [{file,"test/emqx_crl_cache_SUITE.erl"},{line,969}]},
  {test_server,ts_tc,3,[{file,"test_server.erl"},{line,1794}]},
  {test_server,run_test_case_eval1,6,[{file,"test_server.erl"},{line,1303}]},
  {test_server,run_test_case_eval,9,[{file,"test_server.erl"},{line,1235}]}]}
```

Simple to reproduce with a small sleep before `mqtt_connect`:

```diff
modified   src/emqtt.erl
@@ -1032,6 +1032,7 @@ do_connect(ConnMod, #state{pending_calls = Pendings,
                                            socket = NewSock,
                                            pending_calls = NewPendings
                                           }),
+            timer:sleep(50),
             case mqtt_connect(State3) of
                 {ok, State4} ->
                     {ok, State4};
```

Author: thalesmg

src/emqtt.erl

@@ -1034,6 +1034,11 @@ do_connect(ConnMod, #state{pending_calls = Pendings,
                     %% connect on a TLS socket, for example, if the client's TLS
                     %% certificate is revoked and the server closes the connection.
                     {error, closed};
+                {error, {tls_alert, _} = Reason} ->
+                    %% If we receive a TLS alert here such as `Certificate Revoked`, there
+                    %% is no other socket event to be received, and thus we must terminate
+                    %% now to avoid hanging and then getting a `{error, connack_timeout}`.
+                    {error, Reason};
                 {error, Reason} ->
                     ?LOG(info, "failed_to_send_connect_packet", #{reason => Reason}, State),
                     %% Failed to send CONNECT packet.