Merge #687: Load PSBTs using istreambuf_iterator rather than istream_iterator

bb5ea1d qt: Load PSBTs using istreambuf_iterator rather than istream_iterator (Andrew Chow)

Pull request description:

  `istream_iterator` eats whitespace charactesr which causes parsing failures for PSBTs that contain the bytes corresponding to those characters. `istreambuf_iterator` is the correct thing to use here.

  This is a regression in 24.0. bitcoin/bitcoin#25001 accidentally changed the original `istreambuf_iterator` to `istream_iterator`.

ACKs for top commit:
  furszy:
    Tested ACK bb5ea1d
  MarcoFalke:
    review ACK bb5ea1d   🍇

Tree-SHA512: 35d90eee3efdcb6a360af69ac1727f9f2837ea621297196de3136299f5de6d9975df4e425e1fc5b8813c1ddb2a4d60c3969e1d5d968953a4628ca45e37d3bf05

Author: MarcoFalke

src/qt/walletframe.cpp

@@ -212,7 +212,7 @@ void WalletFrame::gotoLoadPSBT(bool from_clipboard)
             return;
         }
         std::ifstream in{filename.toLocal8Bit().data(), std::ios::binary};
-        data.assign(std::istream_iterator<unsigned char>{in}, {});
+        data.assign(std::istreambuf_iterator<char>{in}, {});
 
         // Some psbt files may be base64 strings in the file rather than binary data
         std::string b64_str{data.begin(), data.end()};