Set the overflow flag when the number of bits written reaches the maximum size

[smallmodel]

Currently for example, if bit is 65536 and maxsize is 8192, then cursize would be (65536 >> 3) + 1 = 8193, after the maximum size. (Netchan_Transmit would drop an error when trying to send more than 49152 bytes)
This commit simply sets the overflow flag when the number of bits written reaches the maximum size.

I didn't find better solutions. I first found this one solution:

msg->cursize = (msg->bit>>3);
if (msg->bit & 7)
  msg->cursize++;

However this change must be made in MSG_ReadBits as well, which would break compatibility with existing clients.

[WofWca]

However this change must be made in MSG_ReadBits as well, which would break compatibility with existing clients.

Are you saying that this MR cannot be merged at its current state?

[smallmodel]

The PR is ok, the commit only changes a condition so the number of bits read/written can't be greater than or equal the maximum bit size

[timangus]

Does this fix a bug you've actually experienced, or I guess put another way, how did you come across this?