cgen: fix accessing fields in propagated optional subexpressions (#10136)

vlib/v/gen/c/fn.v

@@ -462,7 +462,7 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 				g.write('\n $cur_line $tmp_opt /*U*/')
 			} else {
 				if !g.inside_const {
-					g.write('\n $cur_line *($unwrapped_styp*)${tmp_opt}.data')
+					g.write('\n $cur_line (*($unwrapped_styp*)${tmp_opt}.data)')
 				} else {
 					g.write('\n $cur_line $tmp_opt')
 				}

vlib/v/tests/option_2_test.v

@@ -56,3 +56,11 @@ fn test_nested_opt() {
 	a := f(f(f(-3) or { -7 }) or { 4 }) or { 17 }
 	assert a == 4
 }
+
+fn foo() ?string {
+	return 'hi'
+}
+
+fn test_opt_subexp_field() ? {
+	assert foo() ?.len == 2
+}