Insecure data handling and null pointer dereferences in test-async.c

@each Coverity identified following problems in the new bin/tests/system/hooks/driver/test-async.c:

*** CID 313488:  Insecure data handling  (TAINTED_SCALAR)
/bin/tests/system/hooks/driver/test-async.c: 345 in async_query_done_begin()
339     	}
340     
341     	/* initial call */
342     	state->async = true;
343     	state->hookpoint = NS_QUERY_DONE_BEGIN;
344     	state->origresult = *resp;
>>>     CID 313488:  Insecure data handling  (TAINTED_SCALAR)
>>>     Passing tainted variable "qctx->client" to a tainted sink.
345     	ns_query_hookasync(qctx, doasync, state);
346     	return (NS_HOOK_RETURN);
347     }
348     
349     static ns_hookresult_t
350     async_qctx_destroy(void *arg, void *cbdata, isc_result_t *resp) {

*** CID 281450:  Null pointer dereferences  (REVERSE_INULL)
/bin/tests/system/hooks/driver/test-async.c: 163 in plugin_register()
157     
158     	*instp = inst;
159     
160     	return (ISC_R_SUCCESS);
161     
162     cleanup:
>>>     CID 281450:  Null pointer dereferences  (REVERSE_INULL)
>>>     Null-checking "inst" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
163     	if (result != ISC_R_SUCCESS && inst != NULL) {
164     		plugin_destroy((void **)&inst);
165     	}
166     
167     	return (result);
168     }

More information at https://scan8.coverity.com/reports.htm#v38342/p12579/fileInstanceId=37570751&defectInstanceId=11302475&mergedDefectId=313488.