Fix two valgrind issues in slab allocator.

During allocation VALGRIND_MAKE_MEM_DEFINED was called with a pointer
as size. That kind of works, but makes valgrind exceedingly slow for
workloads involving the slab allocator.

Secondly there was an access to memory marked as unreachable within
SlabCheck(). Fix that too.

Author: Tomas Vondra
Discussion: https://postgr.es/m/a6543b6d-6015-99b1-63ef-3ed55a76a730@2ndquadrant.com

src/backend/utils/mmgr/slab.c

@@ -403,7 +403,7 @@ SlabAlloc(MemoryContext context, Size size)
 	 * Remove the chunk from the freelist head. The index of the next free
 	 * chunk is stored in the chunk itself.
 	 */
-	VALGRIND_MAKE_MEM_DEFINED(chunk, SlabChunkGetPointer(chunk));
+	VALGRIND_MAKE_MEM_DEFINED(SlabChunkGetPointer(chunk), sizeof(int32));
 	block->firstFreeChunk = *(int32 *) SlabChunkGetPointer(chunk);
 
 	Assert(block->firstFreeChunk >= 0);
@@ -725,6 +725,7 @@ SlabCheck(MemoryContext context)
 
 				/* read index of the next free chunk */
 				chunk = SlabBlockGetChunk(slab, block, idx);
+				VALGRIND_MAKE_MEM_DEFINED(SlabChunkGetPointer(chunk), sizeof(int32));
 				idx = *(int32 *) SlabChunkGetPointer(chunk);
 			}