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
(cherry picked from commit 6a6b91da)
Reviewed-on: https://gerrit.libreoffice.org/16937Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

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");