dd: fix apple gcc return warning (even Go can handle this one, Apple!)

R=rsc
http://codereview.appspot.com/5068043
diff --git a/src/cmd/dd.c b/src/cmd/dd.c
index ad433d8..6f7103e 100644
--- a/src/cmd/dd.c
+++ b/src/cmd/dd.c
@@ -298,6 +298,8 @@
 	c &= 0377;
 	(*conv)(c);
 	goto loop;
+
+	return 0;  // shut up apple gcc
 }
 
 void