Handle lack of date.
 [rsc] --rw-rw-r-- M 320859 glenda sys 31401 Oct 27 10:38 sys/src/cmd/ip/imap4d/msg.c
	/n/sourcesdump/2005/1027/plan9/sys/src/cmd/ip/imap4d/msg.c:747,752 - 
	/n/sourcesdump/2005/1028/plan9/sys/src/cmd/ip/imap4d/msg.c:747,756
	  	s = (char*)headStr;
	  	if(date2tm(&tm, s) == nil)
	  		s = m->info[IUnixDate];
	+ 	if(s == nil){
	+ 		free(ss);
	+ 		goto bogus;
	+ 	}
	  	m->unixDate = estrdup(s);
	  	free(ss);
	  	return 1;