[platform] Speculatively fix fast TLS implementation

... for 32-bit Windows.

When the TLS value is not set the TlsExpansionSlots field of the TEB
struct might be NULL in which case the InternalGetExistingThreadLocal
should return NULL too.

Bug: v8:12653
Change-Id: Ic6c6d8e51e2447551c372720fad0967e9201f3dc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3513835
Auto-Submit: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79437}

src/base/platform/platform.h

@@ -85,7 +85,7 @@ inline intptr_t InternalGetExistingThreadLocal(intptr_t index) {
         __readfsdword(kTibInlineTlsOffset + kSystemPointerSize * index));
   }
   intptr_t extra = static_cast<intptr_t>(__readfsdword(kTibExtraTlsOffset));
-  DCHECK_NE(extra, 0);
+  if (!extra) return 0;
   return *reinterpret_cast<intptr_t*>(extra + kSystemPointerSize *
                                                   (index - kMaxInlineSlots));
 }