ofproto-dpif-xlate: Correct check for MPLS LSE

zero is a valid MPLS LSE so it is not valid check against
that value for MPLS LSE presence. Instead, check against
the flow's dl_type which should be an MPLS type if an LSE is present.

This problem appears to have been introduced by
b2dd70be133bf86c ("Native Set-Field action.").

Cc: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>
ofproto/xlate: Fix set field unwildcarding.

If the field does not exist, nothing is set.  However, we must
unwildcard the bits we used to make the decision, and we need not
unwildcard the field and it's prerequisities, if nothing is set.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>