Ruby - Fixes for Microsoft VC

[cfis]

The two commits fix a MSCV compiler error, compiler warning and crash.

• Commit 63073d3 - MSCV does not support variable length arrays (VLAs), so instead use alloca when using MSVC.

• Commit 1783b9c - Usage of the VOIDPTR_AT macro results in compiler error (or warning) C4146, "unary minus operator applied to unsigned type, result still unsigned". If you manually expand the macro, it becomes (for line 1162):

VOIDPTR_AT(msg, -sizeof(upb_msg_internal))
(void*)(char*)(msg, (int)-sizeof(upb_msg_internal));

size_of returns size_t, which in MSVC is defined as typedef unsigned __int64 size_t. So you are taking the negative value of an unsigned type.

This change simply stops using the macro removing the (int)-sizeof bit causing this issue.

In addition, before this change I would randomly (but frequently) get segmentation faults when loading ruby generated protobuf classes (so just requiring the ruby files, not processing any data). The error was always a pointer at 0xFFFFFFFF. This change fixed the error.

Note I did try and go back to replicate the original crash (I have been running with this patch for about a week), but failed to do.

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[cfis]

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