Merge bitcoin#20303: fuzz: Assert expected DecodeHexTx behaviour when using legacy decoding

d7901ab fuzz: Assert expected DecodeHexTx behaviour when using legacy decoding (practicalswift)

Pull request description:

  Assert expected `DecodeHexTx` behaviour when using legacy decoding.

  As suggested by MarcoFalke in bitcoin#20290 (comment).

ACKs for top commit:
  MarcoFalke:
    review ACK d7901ab

Tree-SHA512: 3285680059e6fa73b0fb2c52b775f6319de1ac616f731206662b742764dc888cdfd1ac1f1fcfdfd5418d2006475a852d1c1a56a7035f772f0a6b2a84f5de93bc

Author: MarcoFalke

src/test/fuzz/decode_tx.cpp

@@ -19,12 +19,14 @@ void test_one_input(const std::vector<uint8_t>& buffer)
     const bool result_none = DecodeHexTx(mtx, tx_hex, false, false);
     const bool result_try_witness = DecodeHexTx(mtx, tx_hex, false, true);
     const bool result_try_witness_and_maybe_no_witness = DecodeHexTx(mtx, tx_hex, true, true);
-    const bool result_try_no_witness = DecodeHexTx(mtx, tx_hex, true, false);
+    CMutableTransaction no_witness_mtx;
+    const bool result_try_no_witness = DecodeHexTx(no_witness_mtx, tx_hex, true, false);
     assert(!result_none);
     if (result_try_witness_and_maybe_no_witness) {
         assert(result_try_no_witness || result_try_witness);
     }
     if (result_try_no_witness) {
+        assert(!no_witness_mtx.HasWitness());
         assert(result_try_witness_and_maybe_no_witness);
     }
 }