QIOSPasteboard: handle zero-pointer argument to setMimeData

QClipboard sends QPlatformClipboard a zero pointer to
QMimeData when it's told to clear. So we need to check
for this to avoid a crash.

Change-Id: I570ed727029ca699673d7b2e989bdff44df8e161
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>

src/plugins/platforms/ios/qiosclipboard.mm

@@ -205,6 +205,10 @@ void QIOSClipboard::setMimeData(QMimeData *mimeData, QClipboard::Mode mode)
     Q_ASSERT(supportsMode(mode));
 
     UIPasteboard *pb = [UIPasteboard pasteboardWithQClipboardMode:mode];
+    if (!mimeData) {
+        pb.items = [NSArray array];
+        return;
+    }
     NSMutableDictionary *pbItem = [NSMutableDictionary dictionaryWithCapacity:mimeData->formats().size()];
 
     foreach (const QString &mimeType, mimeData->formats()) {