Cleanup redundant non-NULL check.

1407        if (sigrdataset != NULL) {
1408                putrdataset(client->mctx, &sigrdataset);
1409        }
    	
CID 288001 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking rctx suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
1410        if (rctx != NULL) {
1411                isc_mutex_destroy(&rctx->lock);
1412                isc_mem_put(mctx, rctx, sizeof(*rctx));
1413        }