oui: minor: Fix comments

Make comments related to skipping of lines a bit clearer.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
dissector: eth: Make port file parsing more robust

Follow commit bdb9efef ("oui: Make parsing of oui.conf more robust") and
make parsing the upd.conf, tcp.conf and ether.conf files more robust
against format flaws. ALso here, in the worst case, we would end up
dereferencing a null pointer.

The null pointer dereference was found by the Coverity scanner.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>