odp-util: Fix segfault in MPLS attribute parsing.

Just because the ethertype is MPLS, this doesn't mean that the datapath
understands and provides OVS_KEY_ATTR_MPLS attributes for the flow.
Previously we would check the size of the OVS_KEY_ATTR_MPLS attribute
before checking whether the attribute is present. This would cause a
segfault in nl_attr_get_size(), usually triggered from a handler thread.

This patch brings the MPLS parsing code more in line with the rest of
the parse_l2_5_onward() function, by only processing MPLS if the
attribute is present.

Reported-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>