Crash if a query string contains stop words

The query parser crashes if terms are being AND'ed
together and the first one is being stopped e.g. "a AND b"

QueryParser::MatchQuery() calls MatchClause() to get
the "a" clause but is being returned NULL because the
word is a stopword. So it doesn't get added to the list
of clauses.

We then get the "b" term and try to AND it with the
first in AddClause() which is assuming that there's
something in the clause list.