odp-util: Don't try to parse geneve data if not provided.

odp-util: Don't try to parse geneve data if not provided.

Despite of linux, 'strtoull' on FreeBSD sets errno to EINVAL in case
of no digits found. This causes odp_flow parsing failure if
there is no geneve data inside it. For example, ovs fails to parse
following flow on FreeBSD:

  tunnel(<...>,geneve({class=0xffff,type=1,len=0}),<...>)

Moving the parsing attempt under the if condition fixes the following
unit test failure:

  tunnel.at:780: testing tunnel - Geneve option present ...
  ./tunnel.at:810: ovs-appctl ofproto/trace ovs-dummy \
      'tunnel(<...>,geneve({class=0xffff,type=1,len=0}), ...'
  --- /dev/null	2018-12-18 13:24:55.001110000 +0000
  +++ /tmp/cirrus-ci-build/tests/testsuite.dir/at-groups/848/stderr
  @@ -0,0 +1,2 @@
  +ovs-dummy: unknown bridge
  +ovs-appctl: ovs-vswitchd: server returned an error

CC: Jesse Gross <jesse@kernel.org>
Fixes: 622a0a8e764d ("odp-util: Geneve netlink decoding.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>