Segmented writes do not work if len(topic) > TX buffer size

[denravonska]

I don't know if this is by design, but "segmented" writes (fill buffer -> write -> fill buffer -> write) does not work if the header and topic length exceeds the TX buffer size when writing. In this crash I tried a TX buffer size of 16 when publishing to abcde/Sally-2.local/status:

frame #1: 0x00000001000a48d7 test `MqttEncode_Publish(tx_buf="����\x7f", tx_buf_len=16, publish=0x00007ffeefbfee28, use_cb='\0') at mqtt_packet.c:842
   839 	        if (payload_len > (tx_buf_len - (header_len + variable_len))) {
   840 	            payload_len = (tx_buf_len - (header_len + variable_len));
   841 	        }
-> 842 	        XMEMCPY(tx_payload, publish->buffer, payload_len);
   843 	    }
   844 	    publish->intBuf_pos = 0;
   845 	    publish->intBuf_len = payload_len;
(lldb) print payload_len
(int) $0 = -16
(lldb) print header_len
(int) $1 = 2
(lldb) print variable_len
(int) $2 = 30
(lldb) print tx_buf_len
(int) $3 = 16

If I increase the buffer size to 32 it works as expected.