Avoid clearing a header list if it is being appended to. I think this resolves a copy/paste error that was introduced in 4a80ae527df9aa36fe50fac0878207a31d4d6b72.
The logic for `message_add_addresses` is that it is supposed to just be appending to a list of addresses, rather than trying to re-merge a series of addresses.
It looks like the function's framing was copy/pasted from `message_update_addresses`, which does the more complete re-build/re-merge.
However, `message_update_addresses` starts with an initial `internet_address_list_clear`, and then proceeds to cycle through the entire list of headers, trying to collect the ones that match the field name, so clearing the address list initially makes sense.
For `message_add_addresses` though, the goal is just to append to the current list that already exists, so the list should not be initially cleared.
