MQTTClient missing implementation to writecontinue

[cuttlfish]

Describe the bug
Seeing a crash when sending large chunks of data > 1MB

To Reproduce
Send large chunk of data using MQTTClient.c instead of async mode. Get this backtrace

#0 0x0000000000000000 in ?? ()
#1 0x0000000000475eac in Socket_continueWrites (sock=0x7fc48969d4,
mutex=0x5ae5b0 <socket_mutex_store>)
at /local/users/dliu/basd3/out.example/_deps/paho-src/src/Socket.c:1412
#2 0x000000000047443c in Socket_getReadySocket (more_work=0, timeout=100,
mutex=0x5ae5b0 <socket_mutex_store>, rc=0x7fc4896a64)
at /local/users/dliu/basd3/out.example/_deps/paho-src/src/Socket.c:535
#3 0x0000000000468fc4 in MQTTClient_cycle (sock=0x7fc4896a60, timeout=100,
rc=0x7fc4896a64)
at /local/users/dliu/basd3/out.example/_deps/paho-src/src/MQTTClient.c:2558
#4 0x0000000000469dcc in MQTTClient_yield ()
at /local/users/dliu/basd3/out.example/_deps/paho-src/src/MQTTClient.c:2828
#5 0x00000000004689e0 in MQTTClient_publish5 (handle=0x2d2c9308,
topicName=0x7fc4896cd0 "bas-d-s/stb/6B8B4567327B23C6/transfer/response/download/CMH2BG9FTEK4NTFB", payloadlen=1051350, payload=0x7f7dcfd010, qos=1,
retained=0, properties=0x7fc4896cb8, deliveryToken=0x7fc4896c8c)
at /local/users/dliu/basd3/out.example/_deps/paho-src/src/MQTTClient.c:2429
#6 0x0000000000468d44 in MQTTClient_publishMessage5 (handle=0x2d2c9308,
topicName=0x7fc4896cd0 "bas-d-s/stb/6B8B4567327B23C6/transfer/response/download/CMH2BG9FTEK4NTFB", message=0x7fc4896c90, deliveryToken=0x7fc4896c8c)
--Type for more, q to quit, c to continue without paging--
at /local/users/dliu/basd3/out.example/_deps/paho-src/src/MQTTClient.c:2500

It seems MQTTAsync.c has a implementation for Socket_setWriteContinueCallback, but MQTTClient.c does not. So when sending large data over MQTTClient, this line crashes

int Socket_continueWrites(SOCKET* sock, mutex_type mutex)
{
...
if (rc == 0)
(*writecontinue)(socket);
...
}

Did not observe this crash in 1.3.10

Expected behavior
Not crash

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Log files
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** Environment (please complete the following information):**

• OS: Linux
• Version 1.3.11

[icraggs]

Fix added to the develop branch.