[compiler-rt] [msan] Detect dereferencing zero-alloc as use-of-uninitialized-value (PR #155944)

[Vitaly Buka via llvm-commits]

================
@@ -230,6 +230,12 @@ static void *MsanAllocate(BufferedStackTrace *stack, uptr size, uptr alignment,
       __msan_set_origin(allocated, size, o.raw_id());
     }
   }
+
+  uptr actually_allocated_size = allocator.GetActuallyAllocatedSize(allocated);
----------------
vitalybuka wrote:

Shouldn't this be like:
```
if (flags()->poison_in_malloc) {
     __msan_poison(allocated + size, allocator.GetActuallyAllocatedSize(allocated) - size);
}
```

https://github.com/llvm/llvm-project/pull/155944