Merge bitcoin#21606: fuzz: Extend psbt fuzz target a bit

faaf395 fuzz: Extend psbt fuzz target a bit (MarcoFalke)

Pull request description:

  Previously it only merged the psbt with itself, now it tries to merge another.

ACKs for top commit:
  practicalswift:
    Tested ACK faaf395

Tree-SHA512: e1b1d31a47d35e1767285bc2fda176c79cb0550d6d383fe467104272e61e1c83f6cbc0c7d6bbc0c3027729eec13ae1f289f8950117ee91e0fb3703e66d5e6918

Author: MarcoFalke

src/test/fuzz/psbt.cpp

@@ -46,6 +46,7 @@ FUZZ_TARGET(psbt)
         (void)PSBTInputSigned(input);
         (void)input.IsNull();
     }
+    (void)CountPSBTUnsignedInputs(psbt);
 
     for (const PSBTOutput& output : psbt.outputs) {
         (void)output.IsNull();