json: fix json.decode bug for u64 values greater than 2^31 (#11238)

vlib/json/json_decode_test.v

@@ -33,3 +33,14 @@ fn test_json_decode_works_with_a_dict_of_arrays() {
 	assert res.twins[1].seed == 'dfgdfgdfgd'
 	assert res.twins[1].pubkey == 'skjldskljh45sdf'
 }
+
+struct Mount {
+	size u64
+}
+
+fn test_decode_u64() ? {
+	data := '{"size": 10737418240}'
+	m := json.decode(Mount, data) ?
+	assert m.size == 10737418240
+	println(m)
+}

vlib/json/json_primitives.v

@@ -96,7 +96,7 @@ fn decode_u64(root &C.cJSON) u64 {
 	if isnil(root) {
 		return u64(0)
 	}
-	return u64(root.valueint)
+	return u64(root.valuedouble)
 }
 
 fn decode_f32(root &C.cJSON) f32 {