wallet2: do not use invalid keys as fake outs in rings

src/wallet/wallet2.cpp

@@ -7019,6 +7019,17 @@ bool wallet2::tx_add_fake_output(std::vector<std::vector<tools::wallet2::get_out
   CHECK_AND_ASSERT_MES(!outs.empty(), false, "internal error: outs is empty");
   if (std::find(outs.back().begin(), outs.back().end(), item) != outs.back().end()) // don't add duplicates
     return false;
+  // check the keys are valid
+  if (!rct::isInMainSubgroup(rct::pk2rct(output_public_key)))
+  {
+    MWARNING("Key " << output_public_key << " at index " << global_index << " is not in the main subgroup");
+    return false;
+  }
+  if (!rct::isInMainSubgroup(mask))
+  {
+    MWARNING("Commitment " << mask << " at index " << global_index << " is not in the main subgroup");
+    return false;
+  }
 //  if (is_output_blackballed(output_public_key)) // don't add blackballed outputs
 //    return false;
   outs.back().push_back(item);