HdlcEncoder::decode lack of validation

[lucasduffey]

https://github.com/openpst/libopenpst/blob/master/src/qualcomm/hdlc_encoder.cpp#L93

// off-by-one OOB read if i == length of data
buffer[osize++] = data[i + 1] ^ HDLC_ESC_MASK;

https://github.com/openpst/libopenpst/blob/master/src/qualcomm/hdlc_encoder.cpp#L102

// int underflow if sizeof(crc) > osize
uint16_t crc = crc16(reinterpret_cast<const char*>(buffer), osize - sizeof(crc));

https://github.com/openpst/libopenpst/blob/master/src/qualcomm/hdlc_encoder.cpp#L103

// OOB read if buffer length < 2
uint16_t chk = *((uint16_t*)&buffer[osize - 2]);