Fix #9361, a2051ba: [Network] Off by one in CanWriteToPacket

[rubidium42]

Motivation / Problem

Fixes #9361. Also partly fixing a2051ba, though it was also wrong before as well in some parts.

The old implementation (pre a2051ba) of Send_uint*/Send_bool function of Packet did assert(this->size < SEND_MTU - sizeof(data));. After a2051ba this effectively became assert(this->size + sizeof(data) < SEND_MTU);, which is still the same functionality wise, and that was the logic CanWriteToPacket was based upon.
However, other functions such as Send_string and custom checks in other places did things like assert(this->size + strlen(data) + 1 <= SEND_MTU);. In a2051ba they lost the = of <=, whereas the Send_uint ones should have actually gained the = as they were wrong/off-by-one.

Note: this is an off-by-one in the safe direction, as it would just prevent the last valid byte of the packet to be used by Send_uint*. It is NOT writing beyond its buffer.

Description

Change bytes_in_packet + bytes_to_add < MAX_BYTES_IN_PACKET to bytes_in_packet + bytes_to_add <= MAX_BYTES_IN_PACKET. Presume bytes_in_packet to be 1, bytes_to_add to be 2 and MAX_BYTES_IN_PACKET to be 3. Then the two bytes can be written to the packet, whereas the previous CanWriteToPacket would not allow it anymore.

Limitations

None

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker