xfrm: fix 'passing zero to ERR_PTR()' warning

xfrm: fix 'passing zero to ERR_PTR()' warning

Fix a static code checker warning:

  net/xfrm/xfrm_policy.c:1836 xfrm_resolve_and_create_bundle() warn: passing zero to 'ERR_PTR'

xfrm_tmpl_resolve return 0 just means no xdst found, return NULL
instead of passing zero to ERR_PTR.

Fixes: d809ec895505 ("xfrm: do not assume that template resolving always returns xfrms")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>