Add assertion check to silence dereference before NULL check in tsig_test.c

363        /*
364         * Start digesting.
365         */
   	deref_ptr_in_call: Dereferencing pointer tsigout. [show details]
366        result = add_mac(outctx, tsigout);
367        assert_int_equal(result, ISC_R_SUCCESS);
...
474        if (tsigin != NULL) {
475                isc_buffer_free(&tsigin);
476        }
   	
CID 281475 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking tsigout suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
477        if (tsigout != NULL) {
478                isc_buffer_free(&tsigout);
479        }