ICE/AV: string mixin of `version =` / `debug =` statement null-derefs in errorSupplemental

[AMDphreak]

DMD hard-crashes with an access violation (0xC0000005 on Windows) when a string mixin contains a statement-level version = ident; or debug = …; declaration.

DMD version

DMD64 D Compiler v2.112.0

Also reproduced on current master (built locally as v2.113.0-beta.1 before the fix).

Steps to reproduce

void main()
{
    mixin("version = foo;");
}

or:

void other()
{
    mixin("debug = 1;");
}

dmd -c -o- crash.d

Expected Behavior

A normal compile error (e.g. version / debug declaration must be at module level, or identifier expected), then non-zero exit without crashing.

Actual Behavior

Process terminates with ACCESS_VIOLATION (0xC0000005, exit code -1073741819 on Windows).

On Windows this shows up in Reliability Monitor / WER as repeated dmd.exe AppCrash entries.

Root cause

1. Parser.parseStatement handles version = / debug = as statements by emitting an error and returning null (same for some failed if parses that used s = null).
2. String-mixin statement semantic then does:

if (!s || global.errors != errors)
{
    errorSupplemental(s.loc, "while parsing string mixin statement");
    ...
}

When !s is true, s.loc is still evaluated → null dereference / AV.

Additional notes

• Searched existing dlang/dmd issues for this mixin/version=/debug= crash pattern; did not find a prior report.
• A fix PR will follow shortly.

[AMDphreak]

Fix PR: #23465