Key pool: Move CanGetAddresses call

Call LegacyScriptPubKeyMan::CanGetAddresses directly instead of calling
CWallet::CanGetAddresses to only query the relevant key manager

This is a minor change in behavior: call now only happens if a new key needs to
be reserved, since if a key is already reserved it might fail unnecessarily.

This change also serves as a sanity check
https://github.com/bitcoin/bitcoin/pull/16341#discussion_r331238394

src/wallet/scriptpubkeyman.cpp

@@ -264,6 +264,10 @@ bool LegacyScriptPubKeyMan::EncryptKeys(CKeyingMaterial& vMasterKeyIn)
 
 bool LegacyScriptPubKeyMan::GetReservedDestination(const OutputType type, bool internal, int64_t& index, CKeyPool& keypool)
 {
+    if (!CanGetAddresses(internal)) {
+        return false;
+    }
+
     if (!ReserveKeyFromKeyPool(index, keypool, internal)) {
         return false;
     }