[wallet] Make setlabel idempotent

Prevent setlabel calls on an address that already has the same label, and which
is the already the default "label address" of that label, from generating a
new default address.

This way calling setlabel on an address that already has the specified label is
a no-op.

Change suggested by Russell Yanofsky <russ@yanofsky.org> in
bitcoin#7729 (comment)

src/wallet/rpcwallet.cpp

@@ -323,7 +323,7 @@ UniValue setlabel(const JSONRPCRequest& request)
         // and if we wouldn't do this, the record would stick around forever.
         if (pwallet->mapAddressBook.count(dest)) {
             std::string old_label = pwallet->mapAddressBook[dest].name;
-            if (dest == GetLabelDestination(pwallet, old_label)) {
+            if (old_label != label && dest == GetLabelDestination(pwallet, old_label)) {
                 DeleteLabel(*pwallet, old_label);
             }
         }

test/functional/wallet_labels.py

@@ -152,9 +152,7 @@ def run_test(self):
         change_label(node, labels[2].addresses[0], labels[2], labels[2])
 
         # Check that setlabel can set the label of an address which is
-        # already the receiving address of the label. It would probably make
-        # sense for this to be a no-op, but right now it resets the receiving
-        # address, causing getlabeladdress to return a brand new address.
+        # already the receiving address of the label. This is a no-op.
         change_label(node, labels[2].receive_address, labels[2], labels[2])
 
 class Label:
@@ -210,7 +208,7 @@ def change_label(node, address, old_label, new_label):
     # address of a different label should reset the receiving address of
     # the old label, causing getlabeladdress to return a brand new
     # address.
-    if address == old_label.receive_address:
+    if old_label.name != new_label.name and address == old_label.receive_address:
         new_address = node.getlabeladdress(old_label.name)
         assert_equal(new_address not in old_label.addresses, True)
         assert_equal(new_address not in new_label.addresses, True)