sss_client: Fix underflow of active_threads
If the memory cache was not initialized and there was a failure in
initialisation of memory cache context (e.g. memory cache file
does not exist) then mc_context had to be destroyed to release
However the count of active threads in sss_cli_mc_ctx is already higher
than zero because current thread is working wih the mc_context.
But this counter was zero-ed with memset in sss_nss_mc_destroy_ctx
due to issue with initialisation of memory cache.
Then we have to decrease counter of active thread in function
sss_nss_mc_get_ctx because initialisation of mc failed.
And the result of this decrement is underflow of counter.
Reviewed-by: Michal Židek <firstname.lastname@example.org>