Fix wrong allocation size in c8f621c4.

In c8f621c4 I forgot to account for MAXALIGN when allocating a new
tuplebuf in ReorderBufferGetTupleBuf(). That happens to currently not
cause active problems on a number of platforms because the affected
pointer is already aligned, but others, like ppc and hppa, trigger this
in the regression test, due to a debug memset clearing memory.

Fix that.

Backpatch: 9.4, like the previous commit.