checkEyecatchers() used to crash if a heap problem was detected

checkEyecatchers() used vsnprintf() with a format specifier "%d" to print the wrong eyecatcher content, but an eyecatcher was defined as 'double'.
Since "%d" expects an 'int' on the stack (which is usually 32 bit in size) but gets a 'double' instead (which is usually 64 bit), the following 'file' argument will be retrieved as wrong pointer value from stack, resulting in a crash in Log() -> vsnprintf() -> strlen().

This was fixed by defining 'eyecatcher' as 'uint64_t' (instead of 'double') and printing an eyecatcher using 'PRIx64' (instead of "d").
Signed-off-by: NAxel Sommerfeldt <axel.sommerfeldt@fastmail.de>

Signed-off-by: NTony Jiang <tony_jansan@aliyun.com>

Encountered this valgrind error while testing some code the other day:

==2568== Memcheck, a memory error detector
==2568== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==2568== Using Valgrind-3.16.1 and LibVEX; rerun with -h for copyright info
==2568== Command: ./test
==2568== Parent PID: 2560
==2568==
==2568== Thread 7:
==2568== Invalid read of size 8
==2568==    at 0x4CDA970: connectionLost_call (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x500AEA6: start_thread (pthread_create.c:477)
==2568==    by 0x5121A2E: clone (clone.S:95)
==2568==  Address 0x91e8238 is 40 bytes inside a block of size 176 free'd
==2568==    at 0x48399AB: free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==2568==    by 0x4CF8B0F: myfree (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x4CF3947: ListUnlink (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x4CF3A16: ListRemove (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x4CDA612: MQTTClient_destroy (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x142BC0: comms_run (comms_connect.c:436)
==2568==    by 0x500AEA6: start_thread (pthread_create.c:477)
==2568==    by 0x5121A2E: clone (clone.S:95)
==2568==  Block was alloc'd at
==2568==    at 0x483877F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==2568==    by 0x4CF8725: mymalloc (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x4CD9F58: MQTTClient_createWithOptions (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x1428FB: comms_run (comms_connect.c:404)
==2568==    by 0x500AEA6: start_thread (pthread_create.c:477)
==2568==    by 0x5121A2E: clone (clone.S:95)
==2568==

It indicates a use after free of the MQTTClient data structure in the
connectionLost_call.

A bit of digging revealed that the problem was that my code Called
MQTTClient_disconnect and MQTTClient_destroy in rapid succession.
Because MQTTClient_disconnect does this:

Thread_start(connectionLost_call, m);

Its entirely possible for connectionLost_call to be executed after the
return from MQTTClient_disconnect completes, at which time the Client
may be destroyed/freed, leading to the above valgrind error.

fix is pretty straightforward. We could use a mutex and condition
variable to serialize the two threads, but Windows doesn't have support
for condition variables currently, so just use one time semaphore to
ensure serialization
Signed-off-by: NNeil Horman <nhorman@gmail.com>

convert to use of semaphore

Merge branch 'mqtt-protocol-url' of https://github.com/fpagliughi/paho.mqtt.c into fpagliughi-mqtt-protocol-url

Merge branch 'readme-updates' of https://github.com/fpagliughi/paho.mqtt.c into fpagliughi-readme-updates

fixed error in comment