Add null check for ThreadLocalStoragePointer in COMUnhandledExceptionFilter to prevent crash on Windows

[Copilot]

Added a null check for NtCurrentTeb()->ThreadLocalStoragePointer at the beginning of COMUnhandledExceptionFilter on Windows to avoid crashing inside coreclr.dll when there is an unhandled exception early during thread initialization. We have similar check in other global EH handlers.

Fixes #115476

[jkotas]

@copilot Revert all changes you have made. Instead, implement the fix mentioned in #115476 (comment)

[Copilot]

@copilot Revert all changes you have made. Instead, implement the fix mentioned in #115476 (comment)

Reverted the previous implementation and implemented a more minimal fix that only protects the GetThreadNULLOk() call with __try/__except. The new approach stores the thread pointer once and reuses it, avoiding multiple TLS access attempts. Changes in commit 0ef7da5.

[jkotas]

@copilot Please revert all changes you have made, DO NOT use any __try/__catch, and just implement the fix that it similar to the code that I have linked to. The fix should early out from COMUnhandledExceptionFilter when NtCurrentTeb()->ThreadLocalStoragePointer is null.

[Copilot]

@copilot Please revert all changes you have made, DO NOT use any __try/__catch, and just implement the fix that it similar to the code that I have linked to. The fix should early out from `COMUnhandle...

Reverted all previous changes and implemented the fix using NtCurrentTeb()->ThreadLocalStoragePointer null check. The function now returns early when TLS is not available, without using any __try/__except. Changes in commit 71288f5.

[Copilot]

Pull request overview

Adds a defensive early-return in COMUnhandledExceptionFilter on Windows when TLS is not yet initialized, preventing a crash when the unhandled exception filter runs during very early thread startup.

Changes:

• Add a Windows-only check for NtCurrentTeb()->ThreadLocalStoragePointer == NULL at the start of COMUnhandledExceptionFilter to avoid touching CLR TLS-backed state too early.