xenstore: send error earlier in do_mkdir

xenstore: send error earlier in do_mkdir

XenServer's coverity instance complains that a few lines below
create_node dereferences NULL if name == NULL. It however fails to
figure out that if node is NULL, errno won't be ENOENT, so do_mkdir
should have bailed before create_node.

That said, it would be good if we don't need to go through the hops.  We
can bail earlier if name is NULL.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>