extended keys: fail to derive too large depth instead of wrapping around

This issue was reported to me by Marco Falke, and found with the
descriptor_parse fuzz target.

src/pubkey.cpp

@@ -365,6 +365,7 @@ void CExtPubKey::DecodeWithVersion(const unsigned char code[BIP32_EXTKEY_WITH_VE
 }
 
 bool CExtPubKey::Derive(CExtPubKey &out, unsigned int _nChild) const {
+    if (nDepth == std::numeric_limits<unsigned char>::max()) return false;
     out.nDepth = nDepth + 1;
     CKeyID id = pubkey.GetID();
     memcpy(out.vchFingerprint, &id, 4);