Merge #13721: Bugfixes for BIP 174 combining and deserialization

fad231ad41 Fix merging of global unknown data in PSBTs (Andrew Chow)
41df035ee1 Check that PSBT keys are the correct length (Andrew Chow)

Pull request description:

  This PR fixes a few bugs that were found and adds tests checking for these errors.

  Specifically:
  - Single byte keys are checked to actually be one byte.
  - Unknown global data must be merged when combining two PSBTs.

Tree-SHA512: c0e7b4bc607d510cc005aaa7c0813ee58c5467ab7ce4adce485522dfeee92b1af3d29fe89df778b0ea812bb3827e085b30e04d4f4ebcefd8364d809573991332

src/script/sign.cpp

@@ -474,6 +474,7 @@ void PartiallySignedTransaction::Merge(const PartiallySignedTransaction& psbt)
     for (unsigned int i = 0; i < outputs.size(); ++i) {
         outputs[i].Merge(psbt.outputs[i]);
     }
+    unknown.insert(psbt.unknown.begin(), psbt.unknown.end());
 }
 
 bool PartiallySignedTransaction::IsSane() const