Add bound checks to utf16_to_utf8

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

status_t Parcel::readString16(String16* pArg) const
{
 size_t len;
 const char16_t* str = readString16Inplace(&len);
 if (str) {
        pArg->setTo(str, len);
 return 0;
 } else {
 *pArg = String16();
 return UNEXPECTED_NULL;
 }
}

status_t Parcel::readString16(String16* pArg) const
{
 size_t len;
 const char16_t* str = readString16Inplace(&len);
 if (str) {
        pArg->setTo(str, len);
 return 0;
 } else {
 *pArg = String16();
 return UNEXPECTED_NULL;
 }
}
