cmd/vet: fix panic in dead code checker on ill-formed switch statements.

A switch statement without a tag requires case values to be bools, but
the parser does not enforce that, so AST-walking code needs to take
care.

Change-Id: I7d9abbb0324314e02a37813c2d2f6adb0d6af5e7
Reviewed-on: https://go-review.googlesource.com/107375Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/dead.go

@@ -45,7 +45,7 @@ func (f *File) updateDead(node ast.Node) {
 				}
 				for _, expr := range cc.List {
 					v := f.pkg.types[expr].Value
-					if v == nil || constant.BoolVal(v) {
+					if v == nil || v.Kind() != constant.Bool || constant.BoolVal(v) {
 						continue BodyLoopBool
 					}
 				}

src/cmd/vet/testdata/deadcode.go

@@ -2123,3 +2123,12 @@ var _ = func() {
 	// goto without label used to panic
 	goto
 }
+
+func _() int {
+	// Empty switch tag with non-bool case value used to panic.
+	switch {
+	case 1:
+		println()
+	}
+	println()
+}