Fix build on native MacOSX (bitcoin#1207)

* Fix build on native MacOSX

In cbc2e5 a bug got introduced while trying to special cases
for Qt >= 5.2

* Use c++11 nullptr literal

src/qt/macdockiconhandler.mm

@@ -15,7 +15,7 @@
 #include <objc/objc.h>
 #include <objc/message.h>
 
-static MacDockIconHandler *s_instance = NULL;
+static MacDockIconHandler *s_instance = nullptr;
 
 bool dockClickHandler(id self,SEL _cmd,...) {
     Q_UNUSED(self)
@@ -31,7 +31,7 @@ void setupDockClickHandler() {
     Class cls = objc_getClass("NSApplication");
     id appInst = objc_msgSend((id)cls, sel_registerName("sharedApplication"));
 
-    if (appInst != NULL) {
+    if (appInst != nullptr) {
         id delegate = objc_msgSend(appInst, sel_registerName("delegate"));
         Class delClass = (Class)objc_msgSend(delegate,  sel_registerName("class"));
         SEL shouldHandle = sel_registerName("applicationShouldHandleReopen:hasVisibleWindows:");
@@ -50,8 +50,8 @@ void setupDockClickHandler() {
     setupDockClickHandler();
     this->m_dummyWidget = new QWidget();
     this->m_dockMenu = new QMenu(this->m_dummyWidget);
-    this->setMainWindow(NULL);
-#elif QT_VERSION >= 0x050200
+    this->setMainWindow(nullptr);
+#if QT_VERSION >= 0x050200
     this->m_dockMenu->setAsDockMenu();
 #endif
     [pool release];
@@ -64,7 +64,7 @@ void setupDockClickHandler() {
 MacDockIconHandler::~MacDockIconHandler()
 {
     delete this->m_dummyWidget;
-    this->setMainWindow(NULL);
+    this->setMainWindow(nullptr);
 }
 
 QMenu *MacDockIconHandler::dockMenu()