Stack and alloca(3)

Hi hackers,

After todays (Mon Jun  9 09:32:45) buildworld, ppp(8) stoped to service my
PPPoE connection. After investigation I found, that the problem lies in lines
519-630 of ether.c.

The problem is, that after memory is allocated with alloca(3), whatever is
sprintf-ed to that frame, is lost. As a quick hack I changed the alloca with
malloc(3) and ppp started to work correctly (I know, it is very nasty hack).

I failed to tigger the bug in any of my test programs. 

Anyone is willing to help me to find the root cause of this bug?

Note: world builded on 6 Jun was working correctly, so it is something in
rather short timeframe.

Cheers,

Wiktor Niesiobedzki