Add bound checks to utf16_to_utf8

Bug: 29250543
Change-Id: I518e7b2fe10aaa3f1c1987586a09b1110aff7e1a
(cherry picked from commit 7e93b2ddcb49b5365fbe1dab134ffb38e6f1c719)

status_t Parcel::readChar(char16_t *pArg) const
{
 int32_t tmp;
 status_t ret = readInt32(&tmp);
 *pArg = char16_t(tmp);
 return ret;
}

status_t Parcel::readChar(char16_t *pArg) const
{
 int32_t tmp;
 status_t ret = readInt32(&tmp);
 *pArg = char16_t(tmp);
 return ret;
}
