contrib/vhost-user-blk: Clean up deallocation of VuVirtqElement

contrib/vhost-user-blk: Clean up deallocation of VuVirtqElement

We allocate VuVirtqElement with g_malloc() in
virtqueue_alloc_element(), but free it with free() in
vhost-user-blk.c.  Harmless, but use g_free() anyway.

One of the calls is guarded by a "not null" condition.  Useless,
because it cannot be null (it's dereferenced right before), and even
it it could be, free() and g_free() do the right thing.  Drop the
conditional.

Fixes: Coverity CID 1490290
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20220630085219.1305519-1-armbru@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>