r770: matching N bases not working properly (#155)

minimap.h

@@ -118,6 +118,7 @@ typedef struct {
 	float min_join_flank_ratio;
 
 	int a, b, q, e, q2, e2; // matching score, mismatch, gap-open and gap-ext penalties
+	int sc_ambi; // score when one or both bases are "N"
 	int noncan;      // cost of non-canonical splicing sites
 	int zdrop, zdrop_inv;   // break alignment if alignment score drops too fast along the diagonal
 	int end_bonus;