When executing an object literal declaration using non-string computed
property name values, the VM crashed caused by an attempt to use a NULL
pointer (result of ucv_string_get() on a non-string value) as hash table
key.

-- Testcase --
{%
	printf("%.J\n", { [1]: "test", [true]: "foo" });
%}
-- End --

-- Expect stdout --
{
	"1": "test",
	"true": "foo"
}
-- End --