We no longer use "packetp" for anything, so eliminate it. (If any

We no longer use "packetp" for anything, so eliminate it.  (If any
dissector really needs source and destination MAC addresses, we should
make global pointers to them - which would be null for packets lacking
MAC addresses, so dissectors that need them will need to do something
sensible if those pointers are null.)

Don't fake up an Ethernet header if there aren't any MAC addresses to
use when faking it up.

"bp_chaddr" in "print-bootp.c" is an array, so "bp->bp_chaddr" cannot be
null, and there's no need to test for it not being null.