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.
int		Huff_getBit( byte *fin, int *offset) {
	int t;
	bloc = *offset;
	t = (fin[(bloc>>3)] >> (bloc&7)) & 0x1;
	bloc++;
	*offset = bloc;
	return t;
}

int		Huff_getBit( byte *fin, int *offset) {
	int t;
	bloc = *offset;
	t = (fin[(bloc>>3)] >> (bloc&7)) & 0x1;
	bloc++;
	*offset = bloc;
	return t;
}
