Key pool: Fix omitted pre-split count in GetKeyPoolSize

This is a bugfix: https://github.com/bitcoin/bitcoin/pull/16341#discussion_r330669214

src/wallet/scriptpubkeyman.cpp

@@ -455,7 +455,7 @@ size_t LegacyScriptPubKeyMan::KeypoolCountExternalKeys()
 unsigned int LegacyScriptPubKeyMan::GetKeyPoolSize() const
 {
     AssertLockHeld(cs_wallet);
-    return setInternalKeyPool.size() + setExternalKeyPool.size();
+    return setInternalKeyPool.size() + setExternalKeyPool.size() + set_pre_split_keypool.size();
 }
 
 int64_t LegacyScriptPubKeyMan::GetTimeFirstKey() const