cmd/yacc: fix compile error in empty grammar

Fixes #12154.

Change-Id: I1e6d1a3479a8a6fc8f53aebd18fb142506110809
Reviewed-on: https://go-review.googlesource.com/13673Reviewed-by: Rob Pike <r@golang.org>

src/cmd/yacc/yacc.go

@@ -3473,6 +3473,7 @@ func ($$rcvr *$$ParserImpl) Parse($$lex $$Lexer) int {
 	var $$lval $$SymType
 	var $$VAL $$SymType
 	var $$Dollar []$$SymType
+	_ = $$Dollar // silence set and not used
 	$$S := make([]$$SymType, $$MaxDepth)
 
 	Nerrs := 0   /* number of errors */