Fix warnings found by the Undefined Behavior Sanitizer

[bjosv]

Fixes runtime error warnings found when running the tests with the undefined behavior sanitizer enabled.

Since the CI-build is not using -fno-sanitize-recover=all the warnings are recoverable and will not fail the CI build.

grep "runtime error" aws-c-mqtt/build/Testing/Temporary/LastTest.log

aws-c-common/source/ref_count.c:29:9: runtime error: call to function s_mqtt_client_connection_start_destroy through pointer to incorrect function type 'void (*)(void *)'
aws-c-common/source/ref_count.c:29:9: runtime error: call to function s_aws_mqtt_client_destroy through pointer to incorrect function type 'void (*)(void *)'

The warnings are also found when a user incorporates aws-sdk-cpp into a project and build it using an UB sanitizer.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.