Fix bug in generation.c's valgrind support.

This doesn't look like the last such bug, but it's one that the
test_decoding regression test is tripping over.  Per buildfarm.

Tomas Vondra

Discussion: https://postgr.es/m/c903f275-2150-fa52-64bf-dca7b53ebf8d@fuzzy.cz

src/backend/utils/mmgr/generation.c

@@ -409,9 +409,14 @@ GenerationAlloc(MemoryContext context, Size size)
 
 	chunk = (GenerationChunk *) block->freeptr;
 
+	/* Prepare to initialize the chunk header. */
+	VALGRIND_MAKE_MEM_UNDEFINED(chunk, Generation_CHUNKHDRSZ);
+
 	block->nchunks += 1;
 	block->freeptr += (Generation_CHUNKHDRSZ + chunk_size);
 
+	Assert(block->freeptr <= block->endptr);
+
 	chunk->block = block;
 
 	chunk->context = set;