Avoid coredump if witness index does not exist

src/wallet/asyncrpcoperation_sendmany.cpp

@@ -428,16 +428,14 @@ bool AsyncRPCOperation_sendmany::main_impl() {
 
         // Add Sapling spends
         for (size_t i = 0; i < saplingNoteInputs_.size(); i++) {
-            //LOCK2(cs_main, pwalletMain->cs_wallet);
-
-            //TODO: avoid coredump here if this index does not exist
-            if (!witnesses[i]) {
+            if (!witnesses.at(i)) {
                 throw JSONRPCError(RPC_WALLET_ERROR,
                     strprintf( "Missing witness for Sapling note at outpoint %s", std::get<0>(saplingNoteInputs_[i]).ToString())
                 );
             }
             if(fZdebug)
                 LogPrintf("%s: Adding Sapling spend\n", __func__);
+
             assert(builder_.AddSaplingSpend(expsk, std::get<1>(saplingNoteInputs_[i]), anchor, witnesses[i].get()));
         }