HPCC-9473 TOPN activity crashes if LIMIT depends on context

CHThorTopNActivity queries the LIMIT in the constructor. However there are
cases where that limit is computed in a parent activity, so it must be queried
later.
This fix moves the query, and the allocation of the buffer, from the ctor
to the ready() method

Signed-off-by: William Whitehead <william.whitehead@lexisnexis.com>