net/rocker: Remove the dead error handling

Memory allocation functions like world_alloc, desc_ring_alloc etc,
they are all wrappers around g_malloc, g_new etc. But g_malloc and
similar functions doesn't return null. Because they ignore the fact
that g_malloc() of 0 bytes returns null. So error checks for these
allocation failure are superfluous. Now, remove them entirely.

Cc: jasowang@redhat.com
Cc: jiri@resnulli.us
Cc: armbru@redhat.com
Cc: f4bug@amsat.org
Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
