Fix agerr() format string issue in chkNum()

Commit 99eda421 fixed agerr() format string issue in yyerror(), but the
same fix is also needed for chkNum().  In chkNum(), format string can be
injected at least via malicious file name:

  $ cat fs4-%n%s%s%s%s%s%s.dot
  graph G { a [ weight = 0g ] }

  $ dot fs4-%n%s%s%s%s%s%s.dot
  Warning: *** %n in writable segment detected ***
  Aborted

lib/cgraph/scan.l

@@ -165,7 +165,7 @@ static int chkNum(void) {
 	agxbput(&xb,buf);
 	agxbput(&xb,fname);
 	agxbput(&xb, " splits into two tokens\n");
-	agerr(AGWARN,agxbuse(&xb));
+	agerr(AGWARN, "%s", agxbuse(&xb));
 
 	agxbfree(&xb);
 	return 1;