[[ Valgrind ]] Ensure marked text is properly initialised.

In some fallback paths, the underlying string would be initialised,
but the start and finish would not be.  This was masked by the
clamping logic, but was detected by Valgrind.
[[ Valgrind ]] Always initialise MCGdkTransferStore pointer before using it.

On handling a system clipboard (etc.) event without owning the
clipboard, it was possible for operations to be carried out on an
uninitialised MCGdkTransferStore pointer.