cmd/goyacc: use a constant instead of zero value

Found with honnef.co/go/tools/cmd/unused.

Change-Id: I373d710e0f5f98dcf591826a26541d1d12696e14
Reviewed-on: https://go-review.googlesource.com/38175
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/goyacc/yacc.go

@@ -2279,7 +2279,7 @@ func output() {
 // temp1[t] is changed to reflect the action
 //
 func precftn(r, t, s int) {
-	var action int
+	action := NOASC
 
 	lp := levprd[r]
 	lt := toklev[t]