Ensure OP_ELITE_SET matches at least some document

[olly]

OP_ELITE_SET should never select groups of terms which don't match any documents. (Currently, will exclude those for which termfreq_max() is 0, but this may still result in a bad choice)

[olly]

Since OP_ELITE_SET performs an OR on the subqueries it selects, this can only be a problem if the selected subqueries are all something like 'a AND b' or 'a NOT b' or NEAR/PHRASE operations, and none of these match anything.

This is pretty obscure, and I'm not sure what the solution is. Perhaps read the first posting from each subquery before picking the elite set to determine if any are in reality empty?