sign: don't assume we are parsing a sane Miniscript

The script provided for signature might be externally provided, for
instance by way of 'finalizepsbt'. Therefore the script might be
ill-crafted, so don't assume pubkeys are always 32 bytes.

Thanks to Niklas for finding this.

src/script/sign.cpp

@@ -295,7 +295,7 @@ struct TapSatisfier: Satisfier<XOnlyPubKey> {
     //! Conversion from a raw xonly public key.
     template <typename I>
     std::optional<XOnlyPubKey> FromPKBytes(I first, I last) const {
-        CHECK_NONFATAL(last - first == 32);
+        if (last - first != 32) return {};
         XOnlyPubKey pubkey;
         std::copy(first, last, pubkey.begin());
         return pubkey;

src/test/script_tests.cpp

@@ -1254,6 +1254,30 @@ BOOST_AUTO_TEST_CASE(script_combineSigs)
     BOOST_CHECK(combined.scriptSig == partial3c);
 }
 
+/**
+ * Reproduction of an exception incorrectly raised when parsing a public key inside a TapMiniscript.
+ */
+BOOST_AUTO_TEST_CASE(sign_invalid_miniscript)
+{
+    FillableSigningProvider keystore;
+    SignatureData sig_data;
+    CMutableTransaction prev, curr;
+
+    // Create a Taproot output which contains a leaf in which a non-32 bytes push is used where a public key is expected
+    // by the Miniscript parser. This offending Script was found by the RPC fuzzer.
+    const auto invalid_pubkey{ParseHex("173d36c8c9c9c9ffffffffffff0200000000021e1e37373721361818181818181e1e1e1e19000000000000000000b19292929292926b006c9b9b9292")};
+    TaprootBuilder builder;
+    builder.Add(0, {invalid_pubkey}, 0xc0);
+    XOnlyPubKey nums{ParseHex("50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0")};
+    builder.Finalize(nums);
+    prev.vout.emplace_back(0, GetScriptForDestination(builder.GetOutput()));
+    curr.vin.emplace_back(COutPoint{prev.GetHash(), 0});
+    sig_data.tr_spenddata = builder.GetSpendData();
+
+    // SignSignature can fail but it shouldn't raise an exception (nor crash).
+    BOOST_CHECK(!SignSignature(keystore, CTransaction(prev), curr, 0, SIGHASH_ALL, sig_data));
+}
+
 BOOST_AUTO_TEST_CASE(script_standard_push)
 {
     ScriptError err;