clear() is reduced

include/mcl/bn.hpp

@@ -1564,17 +1564,22 @@ inline void mulSparse(Fp12& z, const Fp6& x)
 }
 inline void convertFp6toFp12(Fp12& y, const Fp6& x)
 {
-	y.clear();
 	if (BN::param.cp.isMtype) {
 		// (a, b, c) -> (a, c, 0, 0, b, 0)
 		y.a.a = x.a;
 		y.b.b = x.b;
 		y.a.b = x.c;
+		y.a.c.clear();
+		y.b.a.clear();
+		y.b.c.clear();
 	} else {
 		// (a, b, c) -> (b, 0, 0, c, a, 0)
 		y.b.b = x.a;
 		y.a.a = x.b;
 		y.b.a = x.c;
+		y.a.b.clear();
+		y.a.c.clear();
+		y.b.c.clear();
 	}
 }
 inline void mulSparse2(Fp12& z, const Fp6& x, const Fp6& y)