rasdaemon: Fix "ignoring return value" build warning.

rasdaemon: Fix "ignoring return value" build warning.

The following build warning is given:

ras-diskerror-handler.c: In function ras_diskerror_event_handler:
ras-diskerror-handler.c:98:2:
warning: ignoring return value of asprintf, declared with attribute warn_unused_result [-Wunused-result]
  asprintf(&ev.dev, "%u:%u", major(dev), minor(dev));
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Check the return value of asprintf() to avoid the warning.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>