conntrack: Reorder sanity checks in extract_l3_ipvx().

conntrack: Reorder sanity checks in extract_l3_ipvx().

The functions extract_l3_ipv4 and extract_l3_ipv6 check for
unsupported ip fragments and return early.  The checks were after
an assignment that would not be needed when early return happens.
This is slightly inefficient, but mostly reads poorly.
Hence, reorder the ip fragment checks before the assignments.

Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>