xfrm6: Properly handle unsupported protocols

We don't catch the case if an unsupported protocol is submitted
to the xfrm6 protocol handlers, this can lead to NULL pointer
dereferences. Fix this by adding the appropriate checks.

Fixes: 7e14ea15

 ("xfrm6: Add IPsec protocol multiplexer")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>