Fixed ACS scripts executing a function from another library that cause a division/modulus of zero from aborting the game

[AKMDM]

A while ago, I discovered this issue from a Zandronum mod that caused Zandronum to crash, and only looked further into it recently. Currently in GZDoom, this issue causes a fatal error but no crash.

If an ACS script (let's pretend said script is defined in a library called "LIB1") executes a function that's defined in another library (let's also pretend this library is called "LIB2"), and that function causes a division/modulus of zero and forces the script to terminate, the game will abort because of this code:

if (runaway != 0 && InModuleScriptNumber >= 0)
{
	auto scriptptr = activeBehavior->GetScriptPtr(InModuleScriptNumber);
	if (scriptptr != nullptr)
	{
		scriptptr->ProfileData.AddRun(runaway);
	}
	else
	{
		// It is pointless to continue execution. The script is broken and needs to be aborted.
		I_Error("Bad script definition encountered. Script %d is reported running but not present.\nThe most likely cause for this message is using 'delay' inside a function which is not supported.\nPlease check the ACS compiler used for compiling the script!", InModuleScriptNumber);
	}
}

The reason why this happens is because when a function from another library/module is executed, DLevelScript::activeBehavior is temporarily changed to that of the function. There are several or more p-codes that can change the script's state to SCRIPT_DivideBy0 or SCRIPT_ModulusBy0, but they won't revert activeBehavior back to savedActiveBehavior, unlike when the script hits an unknown p-code. As a result, scriptptr will be invalid when the code above executes.

Adding this check just before the code snippet above fixes the error:

// There are several or more p-codes that can trigger a division or modulus of zero.
// Reset the active behavior back to the original if this happens.
if (state == SCRIPT_DivideBy0 || state == SCRIPT_ModulusBy0)
	activeBehavior = savedActiveBehavior;

Here's a link to a minimal example WAD to test:

• Typing pukename test1 in the console executes an ACS script that terminates properly when encountering a division of zero.
• Typing pukename test2 executes an ACS script that causes the game to abort instead without the fix.