pubkey: introduce a GetEvenCorrespondingCPubKey helper

We'll need to get a compressed key out of an x-only one in other places.
Avoid duplicating the code.

src/pubkey.cpp

@@ -204,6 +204,13 @@ std::vector<CKeyID> XOnlyPubKey::GetKeyIDs() const
     return out;
 }
 
+CPubKey XOnlyPubKey::GetEvenCorrespondingCPubKey() const
+{
+    unsigned char full_key[CPubKey::COMPRESSED_SIZE] = {0x02};
+    std::copy(begin(), end(), full_key + 1);
+    return CPubKey{full_key};
+}
+
 bool XOnlyPubKey::IsFullyValid() const
 {
     secp256k1_xonly_pubkey pubkey;

src/pubkey.h

@@ -282,6 +282,8 @@ public:
      */
     std::vector<CKeyID> GetKeyIDs() const;
 
+    CPubKey GetEvenCorrespondingCPubKey() const;
+
     const unsigned char& operator[](int pos) const { return *(m_keydata.begin() + pos); }
     const unsigned char* data() const { return m_keydata.begin(); }
     static constexpr size_t size() { return decltype(m_keydata)::size(); }

src/script/descriptor.cpp

@@ -1401,9 +1401,7 @@ std::unique_ptr<PubkeyProvider> InferPubkey(const CPubKey& pubkey, ParseScriptCo
 
 std::unique_ptr<PubkeyProvider> InferXOnlyPubkey(const XOnlyPubKey& xkey, ParseScriptContext ctx, const SigningProvider& provider)
 {
-    unsigned char full_key[CPubKey::COMPRESSED_SIZE] = {0x02};
+    CPubKey pubkey{xkey.GetEvenCorrespondingCPubKey()};
-    std::copy(xkey.begin(), xkey.end(), full_key + 1);
-    CPubKey pubkey(full_key);
     std::unique_ptr<PubkeyProvider> key_provider = std::make_unique<ConstPubkeyProvider>(0, pubkey, true);
     KeyOriginInfo info;
     if (provider.GetKeyOriginByXOnly(xkey, info)) {