pf: dummynet fix

If we don't have a pipe set we shouldn't feed packets into dummynet.
This could occur if we have a 'dnpipe (0, 100)' configuration, for
example. We do want to feed the packet to dummynet in the return
direction, but not in the forward direction. In that case
pf_pdesc_to_dnflow() should return false, rather than pass a pipe number
of 0 to dummynet.

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