Fix/improve buffer overflow in MSG_ReadBits/MSG_WriteBits

Prevent reading past end of message in MSG_ReadBits. If read past
end of msg->data buffer (16348 bytes) the engine could SEGFAULT.
Make MSG_WriteBits use an exact buffer overflow check instead of
possibly failing with a few bytes left.
void MSG_InitOOB( msg_t *buf, byte *data, int length ) {
	if (!msgInit) {
		MSG_initHuffman();
	}
	Com_Memset (buf, 0, sizeof(*buf));
	buf->data = data;
	buf->maxsize = length;
	buf->oob = qtrue;
}

void MSG_InitOOB( msg_t *buf, byte *data, int length ) {
	if (!msgInit) {
		MSG_initHuffman();
	}
	Com_Memset (buf, 0, sizeof(*buf));
	buf->data = data;
	buf->maxsize = length;
	buf->oob = qtrue;
}
