avoid broken print (Gabriel Diaz)

src/cmd/mk/rule.c

diff --git a/src/cmd/mk/rule.c b/src/cmd/mk/rule.c
index 439d611..3072819 100644
--- a/src/cmd/mk/rule.c
+++ b/src/cmd/mk/rule.c
@@ -74,6 +74,8 @@
 void
 dumpr(char *s, Rule *r)
 {
+	if(r == nil)
+		return;
 	Bprint(&bout, "%s: start=%ld shelltype=%s shellcmd=%s\n", 
 		s, r, r->shellt->name, wtos(r->shellcmd, ' '));
 	for(; r; r = r->next){