ringct: error out when hashToPoint* returns the point at infinity

Reported by QuarksLab.

src/ringct/bulletproofs.cc

@@ -130,7 +130,9 @@ static rct::key get_exponent(const rct::key &base, size_t idx)
 {
   static const std::string salt("bulletproof");
   std::string hashed = std::string((const char*)base.bytes, sizeof(base)) + salt + tools::get_varint_data(idx);
-  return rct::hashToPoint(rct::hash2rct(crypto::cn_fast_hash(hashed.data(), hashed.size())));
+  const rct::key e = rct::hashToPoint(rct::hash2rct(crypto::cn_fast_hash(hashed.data(), hashed.size())));
+  CHECK_AND_ASSERT_THROW_MES(!(e == rct::identity()), "Exponent is point at infinity");
+  return e;
 }
 
 static void init_exponents()

src/ringct/rctSigs.cpp

@@ -277,6 +277,7 @@ namespace rct {
             for (j = 0; j < dsRows; j++) {
                 addKeys2(L, rv.ss[i][j], c_old, pk[i][j]);
                 hashToPoint(Hi, pk[i][j]);
+                CHECK_AND_ASSERT_MES(!(Hi == rct::identity()), false, "Data hashed to point at infinity");
                 addKeys3(R, rv.ss[i][j], Hi, c_old, Ip[j].k);
                 toHash[3 * j + 1] = pk[i][j];
                 toHash[3 * j + 2] = L;