cgen: fix error for empty interface (#13764)

vlib/v/gen/c/infix_expr.v

@@ -507,7 +507,7 @@ fn (mut g Gen) infix_expr_is_op(node ast.InfixExpr) {
 			else { ast.Type(0) }
 		}
 		sub_sym := g.table.sym(sub_type)
-		g.write('_${c_name(sym.name)}_${c_name(sub_sym.name)}_index')
+		g.write('_${sym.cname}_${sub_sym.cname}_index')
 		return
 	} else if sym.kind == .sum_type {
 		g.write('_typ $cmp_op ')

vlib/v/tests/empty_interface_test.v

@@ -0,0 +1,14 @@
+interface Any {}
+
+fn print_out(x Any) string {
+	if x is string {
+		println(x)
+		return '$x'
+	}
+	return ''
+}
+
+fn test_empty_interface() {
+	ret := print_out('12345')
+	assert ret == '&12345'
+}