Better fix for MemoryChunk::owner().

Better fix for MemoryChunk::owner().

Pointer arithmetic such as "owner_ - kFailureTag" is undefined behaviour
unless owner_ points to a valid object.

This allowed Clang to assume the subtraction would never be NULL,
causing problems in the caller (see https://codereview.chromium.org/12090072/).

To fix this, we should cast owner_ to intptr_t before doing the
arithmetic.

Committed: http://code.google.com/p/v8/source/detail?r=13570

[hans, 2013-01-31 15:25:22]

Jakob: PTAL

[Jakob Kummerow, 2013-01-31 15:29:57]

LGTM!

Thanks for investigating this, and also thanks for teaching me some C++!