This prevents an rc suicide when processing an ill-formed here document.

Reference: /n/sources/patch/applied/rc-no-break-on-bad-here-doc
Date: Mon Dec 19 12:16:21 CET 2005

--- /sys/src/cmd/rc/here.c	Mon Dec 19 12:15:46 2005
+++ /sys/src/cmd/rc/here.c	Mon Dec 19 12:15:43 2005
@@ -62,7 +62,7 @@
 		while((c = rchr(runq->cmdfd))!=EOF){
 			if(c=='\n' || s==&line[NLINE]){
 				*s='\0';
-				if(strcmp(line, tag)==0) break;
+				if(tag && strcmp(line, tag)==0) break;
 				if(subst)
 					psubst(f, line);
 				else pstr(f, line);