xfrm Fix potential error pointer dereference in xfrm_bundle_create.

We may derference an invalid pointer in the error path of
xfrm_bundle_create(). Fix this by returning this error
pointer directly instead of assigning it to xdst0.

Fixes: 45b018be

 ("ipsec: Create and use new helpers for dst child access.")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>