The list of input tables in filesort is currently represented by
Prealloced_array<TABLE *, 4>. However, when going over the limit
(ie., when grouping a join of five or more tables), it starts
allocating memory from the heap, _not_ the MEM_ROOT.

This causes memory leaks, since Filesort objects are not always
destroyed. Change the table list to Mem_root_array<TABLE *> instead.

Change-Id: I0cf046194f7a78816b12e77e169e00f52b358f1d