xfrm: Fix error reporting in xfrm_state_construct.

When memory allocation for XFRMA_ENCAP or XFRMA_COADDR fails,
the error will not be reported because the -ENOMEM assignment
to the err variable is overwritten before. Fix this by moving
these two in front of the function so that memory allocation
failures will be reported.

Reported-by: Tobias Brunner <tobias@strongswan.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>