field extension supposes non-fullbit

include/mcl/fp_tower.hpp

@@ -675,6 +675,7 @@ struct Fp2DblT {
 	void operator-=(const Fp2DblT& x) { sub(*this, *this, x); }
 	static void init()
  	{
+		assert(!Fp::getOp().isFullBit);
 		const mcl::fp::Op& op = Fp::getOp();
 		if (op.fp2Dbl_mulPreA_) {
 			mulPre = fp::func_ptr_cast<void (*)(Fp2DblT&, const Fp2&, const Fp2&)>(op.fp2Dbl_mulPreA_);

test/fp_tower_test.cpp

@@ -401,6 +401,7 @@ void test(const char *p, mcl::fp::Mode mode)
 	const int xi_a = 1;
 	Fp::init(xi_a, p, mode);
 	printf("mode=%s\n", mcl::fp::ModeToStr(mode));
+	if (Fp::getOp().isFullBit) return;
 	Fp2::init();
 	printf("bitSize=%d\n", (int)Fp::getBitSize());
 #if 0