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;