OpenFlow: Add an overshoot guard back.

In openflow_print() terminate the loop if the downstream code managed
to run off the TCP payload end without running off the packet buffer
end (that's how the pointers worked before commit ad69daa2, but I got
the recent conversion to a decrementing unsigned length wrong in commit
4e2e9c24). Expand the comment further.

Declare OF_HEADER_FIXLEN as unsigned to squelch a signedness warning.