More concise conversion of CDataStream to string

Use .str() instead of .data() and .size() when converting CDataStream to
a string. Uses std::string, avoiding conversion to a C string.

src/rpc/rawtransaction.cpp

@@ -1554,10 +1554,10 @@ UniValue finalizepsbt(const JSONRPCRequest& request)
             mtx.vin[i].scriptWitness = psbtx.inputs[i].final_script_witness;
         }
         ssTx << mtx;
-        result.pushKV("hex", HexStr(ssTx.begin(), ssTx.end()));
+        result.pushKV("hex", HexStr(ssTx.str()));
     } else {
         ssTx << psbtx;
-        result.pushKV("psbt", EncodeBase64((unsigned char*)ssTx.data(), ssTx.size()));
+        result.pushKV("psbt", EncodeBase64(ssTx.str()));
     }
     result.pushKV("complete", complete);
 

src/wallet/rpcwallet.cpp

@@ -3842,7 +3842,7 @@ UniValue walletprocesspsbt(const JSONRPCRequest& request)
     UniValue result(UniValue::VOBJ);
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
     ssTx << psbtx;
-    result.pushKV("psbt", EncodeBase64((unsigned char*)ssTx.data(), ssTx.size()));
+    result.pushKV("psbt", EncodeBase64(ssTx.str()));
     result.pushKV("complete", complete);
 
     return result;
@@ -3956,7 +3956,7 @@ UniValue walletcreatefundedpsbt(const JSONRPCRequest& request)
     ssTx << psbtx;
 
     UniValue result(UniValue::VOBJ);
-    result.pushKV("psbt", EncodeBase64((unsigned char*)ssTx.data(), ssTx.size()));
+    result.pushKV("psbt", EncodeBase64(ssTx.str()));
     result.pushKV("fee", ValueFromAmount(fee));
     result.pushKV("changepos", change_position);
     return result;