dlt-user: Fix crashes in dlt_free during dlt_init (#362)

If dlt_free is called while dlt_init is running, the dlt-user library
crashes because the init and free where not thread safe.
This commit introduces a new field dlt_user_initialising
which will prevent entering dlt_free while dlt_init is running.
Correctness is ensured by a unit test

Signed-off-by: Alexander Mohr <[email protected]>

Author: alexmohr