prv_parseItem() in json.c use string as a boolean

[stefanfangvall]

string is no boolean, do not use negation

static int prv_parseItem(const uint8_t * buffer,
size_t bufferLen,
_record_t * recordP)
{
.
.

case _TYPE_STRING:
if (0 != recordP->valueLen)
{
size_t stringLen;
uint8_t *string = (uint8_t *)lwm2m_malloc(recordP->valueLen);
if (!string) return false;
stringLen = json_unescapeString(string, recordP->value, recordP->valueLen);
if (stringLen)
{
lwm2m_data_encode_nstring((char *)string, stringLen, targetP);
lwm2m_free(string);
}