fuzz: make sure PSBT serialization roundtrips

This will prevent us from creating a serialization we do not accept
going forward.

src/test/fuzz/psbt.cpp

@@ -33,6 +33,17 @@ FUZZ_TARGET(psbt)
     }
     const PartiallySignedTransaction psbt = psbt_mut;
 
+    // A PSBT must roundtrip.
+    PartiallySignedTransaction psbt_roundtrip;
+    std::vector<uint8_t> psbt_ser;
+    VectorWriter{psbt_ser, 0, psbt};
+    SpanReader{psbt_ser} >> psbt_roundtrip;
+
+    // And be stable across roundtrips.
+    std::vector<uint8_t> roundtrip_ser;
+    VectorWriter{roundtrip_ser, 0, psbt_roundtrip};
+    Assert(psbt_ser == roundtrip_ser);
+
     const PSBTAnalysis analysis = AnalyzePSBT(psbt);
     (void)PSBTRoleName(analysis.next);
     for (const PSBTInputAnalysis& input_analysis : analysis.inputs) {