Ensure err is a useful value to prevent using fputs() with a NULL stream.

MFC after:	1 week

usr.bin/pr/pr.c

@@ -1590,6 +1590,7 @@ setup(argc, argv)
 		 * defer diagnostics until processing is done
 		 */
 		if ((err = tmpfile()) == NULL) {
+		       err = stderr;
 		       (void)fputs("Cannot defer diagnostic messages\n",stderr);
 		       return(1);
 		}