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/key.cpp

@@ -333,6 +333,7 @@ bool CKey::Derive(CKey& keyChild, ChainCode &ccChild, unsigned int nChild, const
 }
 
 bool CExtKey::Derive(CExtKey &out, unsigned int _nChild) const {
+    if (nDepth == std::numeric_limits<unsigned char>::max()) return false;
     out.nDepth = nDepth + 1;
     CKeyID id = key.GetPubKey().GetID();
     memcpy(out.vchFingerprint, &id, 4);