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_WriteAngle16( msg_t *sb, float f ) {
	MSG_WriteShort (sb, ANGLE2SHORT(f));
}

void MSG_WriteAngle16( msg_t *sb, float f ) {
	MSG_WriteShort (sb, ANGLE2SHORT(f));
}
