Don't use zero as null pointer constant (-Wzero-as-null-pointer-constant)

Qt-only changes.

src/qt/editaddressdialog.cpp

@@ -15,9 +15,9 @@
 EditAddressDialog::EditAddressDialog(Mode _mode, QWidget *parent) :
     QDialog(parent),
     ui(new Ui::EditAddressDialog),
-    mapper(0),
+    mapper(nullptr),
     mode(_mode),
-    model(0)
+    model(nullptr)
 {
     ui->setupUi(this);