CID 356329: Error handling issues in bin/dnssec/dnssec-signzone.c

Coverity Scan suggests to check return value of isc__task_create() (originated in b69e7831) as in other places:

/bin/dnssec/dnssec-signzone.c: 3732 in main()
3726     	if (directory == NULL) {
3727     		directory = ".";
3728     	}
3729     
3730     	isc_managers_create(&mctx, ntasks, &loopmgr, &netmgr, &taskmgr);
3731     
>>>     CID 356329:  Error handling issues  (CHECKED_RETURN)
>>>     Calling "isc__task_create" without checking return value (as is done elsewhere 16 out of 18 times).
3732     	isc_task_create(taskmgr, &write_task, 0);
3733     
3734     	result = dst_lib_init(mctx, engine);
3735     	if (result != ISC_R_SUCCESS) {
3736     		fatal("could not initialize dst: %s",
3737     		      isc_result_totext(result));