Further improvements to c8f621c43.

Further improvements to c8f621c43.

Coverity and inspection for the issue addressed in fd45d16f found some
questionable code.

Specifically coverity noticed that the wrong length was added in
ReorderBufferSerializeChange() - without immediate negative consequences
as the variable isn't used afterwards.  During code-review and testing I
noticed that a bit of space was wasted when allocating tuple bufs in
several places.  Thirdly, the debug memset()s in
ReorderBufferGetTupleBuf() reduce the error checking valgrind can do.

Backpatch: 9.4, like c8f621c43.