pppoe: Avoid double free (thanks k0ga)

k0ga reports:

Hello,

While I was setting my pppoe conexion with my ISP
I discovered several problems in ip/pppoe. I used
the command line ip/pppoe -A '' ether0 and I got
this:

...
dropping unwanted pkt: wrong ac name
panic: D2B called on non-block dc10 (double-free?)
note rcved: sys: trap: fault read addr=0x0 pc=0x000066e1
pppoe 1013: suicide: sys: trap: fault read addr=0x0 pc=0x000066e1
cpu% acid 1013
/proc/1013/text:386 plan 9 executable
/sys/lib/acid/port
/sys/lib/acid/386
acid: stk()
abort()+0x0 /sys/src/libc/9sys/abort.c:6
ppanic(p=0xd1b8,fmt=0xc7f9)+0x146 /sys/src/libc/port/malloc.c:166
D2B(p=0xd1b8,v=0xdc10)+0x57 /sys/src/libc/port/pool.c:926
poolfreel(v=0xdc10,p=0xd1b8)+0x20 /sys/src/libc/port/pool.c:1152
poolfree(p=0xd1b8,v=0xdc10)+0x3b /sys/src/libc/port/pool.c:1287
free(v=0xdc18)+0x23 /sys/src/libc/port/malloc.c:250
clearstate()+0x1b /sys/src/cmd/ip/pppoe.c:328
pppoe(ether=0xdfffefc1)+0x123 /sys/src/cmd/ip/pppoe.c:426
main(argv=0xdfffefa0,argc=0x1)+0x89 /sys/src/cmd/ip/pppoe.c:100
_main+0x31 /sys/src/libc/386/main9.s:16
acid:


clearstate() is called in pppoe.c:424, and it frees acname and sets it
to nil.  pktread() is called in pppoe.c:434 with parameter wantoffer,
which frees acname again in line pppoe.c:360 but doesn't set it to
nil, so clearstate() makes a double free in the next iteration.

sys/src/cmd/ip/pppoe.c

@@ -358,6 +358,7 @@ wantoffer(uchar *pkt)
 	acname = copy(s, len);
 	if(wantac && strcmp(acname, wantac) != 0){
 		free(acname);
+		acname = nil;
 		return bad("wrong ac name");
 	}