Check that user supplied the required argument; and if not, show usage().

Obtained from:	OpenBSD

usr.bin/yacc/yyfix.sh

@@ -37,6 +37,11 @@ OLDYACC="yyexca yyact yypact yypgo yyr1 yyr2 yychk yydef"
 NEWYACC="yylhs yylen yydefred yydgoto yysindex yyrindex yygindex \
 	 yytable yycheck"
 
+if [ $# -eq 0 ]; then
+	echo "usage: $0 file [tables]" >&2
+	exit 1
+fi
+
 file=$1
 >$file
 shift