Ensure that cache pointer is set to NULL by isc_tlsctx_cache_detach()
If the reference count was higher than 1, detaching a tlsctx cache didn't clear the pointer, which could trigger an assertion later.
Closes #3075 (closed)
If the reference count was higher than 1, detaching a tlsctx cache didn't clear the pointer, which could trigger an assertion later.
Closes #3075 (closed)