partial bitcoin#21606: Extend psbt fuzz target a bit

excludes CountPSBTUnsignedInputs from the PSBT fuzzing test series

src/test/fuzz/psbt.cpp

@@ -2,13 +2,15 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <test/fuzz/FuzzedDataProvider.h>
 #include <test/fuzz/fuzz.h>
 
 #include <node/psbt.h>
 #include <psbt.h>
 #include <pubkey.h>
 #include <script/script.h>
 #include <streams.h>
+#include <util/check.h>
 #include <version.h>
 
 #include <cstdint>
@@ -18,10 +20,10 @@
 
 FUZZ_TARGET(psbt)
 {
+    FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
     PartiallySignedTransaction psbt_mut;
-    const std::string raw_psbt{buffer.begin(), buffer.end()};
     std::string error;
-    if (!DecodeRawPSBT(psbt_mut, raw_psbt, error)) {
+    if (!DecodeRawPSBT(psbt_mut, fuzzed_data_provider.ConsumeRandomLengthString(), error)) {
         return;
     }
     const PartiallySignedTransaction psbt = psbt_mut;
@@ -68,6 +70,20 @@ FUZZ_TARGET(psbt)
         const PartiallySignedTransaction psbt_from_tx{result};
     }
 
+    PartiallySignedTransaction psbt_merge;
+    if (!DecodeRawPSBT(psbt_merge, fuzzed_data_provider.ConsumeRandomLengthString(), error)) {
+        psbt_merge = psbt;
+    }
+    psbt_mut = psbt;
+    (void)psbt_mut.Merge(psbt_merge);
+    psbt_mut = psbt;
+    (void)CombinePSBTs(psbt_mut, {psbt_mut, psbt_merge});
     psbt_mut = psbt;
-    (void)psbt_mut.Merge(psbt);
+    for (unsigned int i = 0; i < psbt_merge.tx->vin.size(); ++i) {
+        (void)psbt_mut.AddInput(psbt_merge.tx->vin[i], psbt_merge.inputs[i]);
+    }
+    for (unsigned int i = 0; i < psbt_merge.tx->vout.size(); ++i) {
+        Assert(psbt_mut.AddOutput(psbt_merge.tx->vout[i], psbt_merge.outputs[i]));
+    }
+    psbt_mut.unknown.insert(psbt_merge.unknown.begin(), psbt_merge.unknown.end());
 }