factotum: fix nil pointer crash on p9skclosekey(), thanks k0ga
oversight, closekey is called again even if addkey failed, which
will have Key.priv nil.
stack trace:
memset(data=0x0,n=0x1f8)+0x58 /sys/src/libc/arm/memset.s:40
p9skclosekey(k=0x55358)+0x18 /sys/src/cmd/auth/factotum/p9sk1.c:493
closekey(k=0x55358)+0x7c /sys/src/cmd/auth/factotum/util.c:247
ctlwrite(a=0x552f8,atzero=0x0)+0x320 /sys/src/cmd/auth/factotum/rpc.c:503
fswrite(r=0x53b78)+0x1b0 /sys/src/cmd/auth/factotum/fs.c:574
swrite(r=0x53b78,srv=0x423e8)+0x134 /sys/src/lib9p/srv.c:559
srvwork()+0x2a4 /sys/src/lib9p/srv.c:746
srv(srv=0x423e8)+0xcc /sys/src/lib9p/srv.c:825
postproc()+0x3c /sys/src/lib9p/post.c:120
rforker(flag=0x1,fn=0x39f98,arg=0x423e8)+0x34 /sys/src/lib9p/rfork.c:16
_postmountsrv(s=0x423e8,name=0x0,mtpt=0x46f70,flag=0x1)+0xb8 /sys/src/lib9p/post.c:27
postmountsrv(name=0x0,mtpt=0x46f70,flag=0x1)+0x20 /sys/src/lib9p/rfork.c:32
main(argv=0x1fffff9c,argc=0x0)+0x23c /sys/src/cmd/auth/factotum/fs.c:157
_main+0x28 /sys/src/libc/arm/main9.s:19
acid: /sys/src/cmd/auth/factotum/p9sk1.c:493
faecd86ee6