Fix MatchVector::first_token to ignore negative values.

It was looking for minimum value and Position::Nowhere is
the smallest you can get value-wise (min_int). So if there was
an empty Match in a vector, first_token for the vector would
always end up returning Position::Nowhere, which was wrong
(empty submatches should've been ignored and Position::Nowhere
returned only if there were no non-empty alternatives).

libwccl/values/matchvector.cpp

@@ -27,12 +27,21 @@ Position MatchVector::first_token(const boost::shared_ptr<Corpus2::AnnotatedSent
 	if (matches_.empty()) {
 		return Position(Position::Nowhere);
 	} else {
+		// Negative positions are invalid, including specials like Nowhere,
+		// so we can't just find minimum value but minimum *non-negative* value.
+		// Note: yes, the code assumes the special values like Nowhere are indeed negative.
 		Position p = matches_.front()->first_token(s);
-		for (size_t i = 1; i < matches_.size(); ++i) {
+		size_t i = 1;
+		while ((p.get_value() < 0) && (i < matches_.size())) {
+			p = matches_[i]->first_token(s);
+			++i;
+		}
+		while (i < matches_.size()) {
 			Position c = matches_[i]->first_token(s);
-			if (c.get_value() < p.get_value()) {
+			if ((c.get_value() >= 0) && (c.get_value() < p.get_value())) {
 				p = c;
 			}
+			++i;
 		}
 		return p;
 	}

tests/match.cpp

@@ -98,6 +98,18 @@ BOOST_AUTO_TEST_CASE(matchvector_first_last)
 	m1->append(boost::make_shared<TokenMatch>(1));
 	BOOST_CHECK_EQUAL(m2.first_token(ptr).get_value(), 1);
 	BOOST_CHECK_EQUAL(m2.last_token(ptr).get_value(), 7);
+	Wccl::MatchVector m3;
+	BOOST_CHECK_EQUAL(m3.to_raw_string(), "MATCH()");
+	BOOST_CHECK_EQUAL(m3.first_token(ptr).get_value(), Wccl::Position::Nowhere);
+	BOOST_CHECK_EQUAL(m3.last_token(ptr).get_value(), Wccl::Position::Nowhere);
+	m3.append(boost::make_shared<MatchVector>());
+	BOOST_CHECK_EQUAL(m3.to_raw_string(), "MATCH(MATCH())");
+	BOOST_CHECK_EQUAL(m3.first_token(ptr).get_value(), Wccl::Position::Nowhere);
+	BOOST_CHECK_EQUAL(m3.last_token(ptr).get_value(), Wccl::Position::Nowhere);
+	m3.append(boost::make_shared<TokenMatch>(1));
+	BOOST_CHECK_EQUAL(m3.to_raw_string(), "MATCH(MATCH(),TOK[1])");
+	BOOST_CHECK_EQUAL(m3.first_token(ptr).get_value(), 1);
+	BOOST_CHECK_EQUAL(m3.last_token(ptr).get_value(), 1);
 }
 
 BOOST_AUTO_TEST_CASE(varmatch)