bug in SlabAlloc / VALGRIND_MAKE_MEM_DEFINED

Hi,

Andres nagged to me about valgrind runs taking much longer since 
9fab40ad introduced the SlabContext into reorderbuffer.c. And by 
"longer" I mean hours instead of minutes.

After a bit of investigation I stumbled on this line in slab.c:

   VALGRIND_MAKE_MEM_DEFINED(chunk, SlabChunkGetPointer(chunk));

which is wrong, because the second parameter should be size and not 
another pointer. This essentially marks a lot of memory as defined, 
which results in the extreme test runtime.

Instead, the line should be:

   VALGRIND_MAKE_MEM_DEFINED(SlabChunkGetPointer(chunk), sizeof(int32));

Patch attached.

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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