[9fans] libc/strcmp bug?

["Skip Tavakkolian" <fst@centurytel.net>]

I believe there is a bug in /sys/src/libc/port/strcmp.c, because it
does not check for null pointers getting passed in before
dereferencing them.  I ran into it when attempting to Post some mail;
'marshal' dies with this message:

marshal 2014: suicide: sys: trap: fault read addr=0x0 pc=0x00006feb

Inspection points to the strcmp:

term% acid 2014
/proc/2014/text:386 plan 9 executable

/sys/lib/acid/port
/sys/lib/acid/386
acid: stk()
At pc:0x00006feb:strcmp+0xe /sys/src/libc/port/strcmp.c:10
strcmp(s1=0x00000000,s2=0x00013c5a) /sys/src/libc/port/strcmp.c:5
	called from islikeatty+0x32 /sys/src/cmd/upas/common/libsys.c:709
islikeatty(fd=0x00000000) /sys/src/cmd/upas/common/libsys.c:701
	called from holdon+0xf /sys/src/cmd/upas/common/libsys.c:719
holdon() /sys/src/cmd/upas/common/libsys.c:715
	called from main+0x856 /sys/src/cmd/upas/marshal/marshal.c:271
main(argv=0x7fffefec,argc=0x00000000) /sys/src/cmd/upas/marshal/marshal.c:162
	called from _main+0x31 /sys/src/libc/386/main9.s:16
acid:

I'm not sure why the first arg to strcmp was null.  I'm still investigating.