fix SentenceContext::at(int) UB due to wrong check being done

libwccl/sentencecontext.h

@@ -75,7 +75,7 @@ public:
 	/// Token access convenience function - const.
 	/// Will return NULL if the passed position is not valid in this Sentence
 	const Corpus2::Token* at(int abs_pos) const {
-		if (is_current_inside()) {
+		if (is_inside(abs_pos)) {
 			return get_sentence()[abs_pos];
 		} else {
 			return NULL;
@@ -90,7 +90,7 @@ public:
 	/// Token access convenience function.
 	/// Will return NULL if the passed position is not valid in this Sentence
 	Corpus2::Token* at(int abs_pos) {
-		if (is_current_inside()) {
+		if (is_inside(abs_pos)) {
 			return get_sentence()[abs_pos];
 		} else {
 			return NULL;