ipvs: ip_vs_sh: ip_vs_sh_get_port: check skb_header_pointer for NULL

skb_header_pointer could return NULL, so check for it as we do it
everywhere else in ipvs code. This fixes a coverity warning.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>