A less-intrusive fix: don't have table_cond_selectivity() assume that
there are less than MAX_REF_PARTS hash-join KEYUSEs.

If there are more than that, switch to using an array. Allocate the array
on the heap: we can't allocate it on MEM_ROOT as table_cond_selectivity()
is called many times during the optimization.

(Variant 2, with review input addressed)