fix clang warnings reported by Tuncer Ayaz

R=rsc
http://codereview.appspot.com/6744054

src/cmd/auth/dsasign.c

@@ -146,6 +146,7 @@ start:
 		fmtprint(&fmt, "%s\n", p);
 	}
 	sysfatal("did not find end of message");
+	return; // silence clang warning
 
 end:
 	text = fmtstrflush(&fmt);

src/cmd/auth/factotum/chap.c

@@ -122,6 +122,7 @@ chapclient(Conv *c)
 	chal = nil;
 	k = nil;
 	attr = c->attr;
+	res = nil;
 
 	if(c->proto == &chap){
 		astype = AuthChap;