vcl: fix deadlock in VclGtkClipboard::setContents()

Must not call getTransferDataFlavors with m_Mutex held;

6  TransferableHelper::getTransferDataFlavors() (this=0x567b980) at svtools/source/misc/transfer.cxx:392
7  VclGtkClipboard::setContents(com::sun::star::uno::Reference<com::sun::star::datatransfer::XTransferable> const&, com::sun::star::uno::Reference<com::sun::star::datatransfer::clipboard::XClipboardOwner> const&) (this=0x3199550, xTrans=uno::Reference to (SwTransferable *) 0x567b9a8, xClipboardOwner=uno::Reference to (SwTransferable *) 0x567b9b0) at vcl/unx/gtk3_kde5/../gtk3/gtk3gtkinst.cxx:618
8  TransferableHelper::CopyToSelection(vcl::Window*) const (this=0x567b980, pWindow=0x301b0c0) at svtools/source/misc/transfer.cxx:960

CopyToSelection() uses SolarMutexReleaser, then getTransferDataFlavors()
has a SolarMutexGuard; this will deadlock against the main thread
that holds SolarMutex and calls VclGtkClipboard::setContents() itself,
from VclGtkClipboard::OwnerPossiblyChanged().

Change-Id: Ibda0b6bce6f3388c45cee8077dd977abb3dda366
Reviewed-on: https://gerrit.libreoffice.org/67121
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

vcl/unx/gtk3/gtk3gtkinst.cxx

@@ -617,6 +617,12 @@ void VclGtkClipboard::setContents(
         const Reference< css::datatransfer::XTransferable >& xTrans,
         const Reference< css::datatransfer::clipboard::XClipboardOwner >& xClipboardOwner )
 {
+    css::uno::Sequence<css::datatransfer::DataFlavor> aFormats;
+    if (xTrans.is())
+    {
+        aFormats = xTrans->getTransferDataFlavors();
+    }
+
     osl::ClearableMutexGuard aGuard( m_aMutex );
     Reference< datatransfer::clipboard::XClipboardOwner > xOldOwner( m_aOwner );
     Reference< datatransfer::XTransferable > xOldContents( m_aContents );
@@ -635,7 +641,6 @@ void VclGtkClipboard::setContents(
     assert(m_aGtkTargets.empty());
     if (m_aContents.is())
     {
-        css::uno::Sequence<css::datatransfer::DataFlavor> aFormats = xTrans->getTransferDataFlavors();
         std::vector<GtkTargetEntry> aGtkTargets(m_aConversionHelper.FormatsToGtk(aFormats));
         if (!aGtkTargets.empty())
         {