rtl8139: drop tautologous if (ip) {...} statement (CVE-2015-5165)

The previous patch stopped using the ip pointer as an indicator that the
IP header is present.  When we reach the if (ip) {...} statement we know
ip is always non-NULL.

Remove the if statement to reduce nesting.

Reported-by: 朱东海(启路) <donghai.zdh@alibaba-inc.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
