fix assert when pasting nil clipboard content

This happened when one of our modules had clipboard content and was
closed, which currently clears the clipboard, and then reopened and
attempting to paste the content. Apparently the utf-16 mime-type is
still alive but the content isn't, on which rtl_str_getLength(pText)
asserts.

Change-Id: I5cc7c20ac1f1a8551a10b033e1f37bf83c5cc202

vcl/unx/gtk3/app/gtk3gtkinst.cxx

@@ -112,9 +112,10 @@ public:
         GtkClipboard* clipboard = gtk_clipboard_get(GDK_SELECTION_CLIPBOARD);
         if (rFlavor.MimeType == "text/plain;charset=utf-16")
         {
+            OUString aStr;
             gchar *pText = gtk_clipboard_wait_for_text(clipboard);
-            OUString aStr(pText, rtl_str_getLength(pText),
-                RTL_TEXTENCODING_UTF8);
+            if (pText)
+                aStr = OUString(pText, rtl_str_getLength(pText), RTL_TEXTENCODING_UTF8);
             g_free(pText);
             css::uno::Any aRet;
             aRet <<= aStr.replaceAll("\r\n", "\n");