Assert selected_partition_ptr is valid before use (#750168)

Further ensure that a bug doesn't get introduced with the use of
selected_partition_ptr, by asserting that it points to a current
partition object in the vector of display partitions.

After deliberately breaking the code so that selected_partition_ptr
points to some other partition object, trying to display the Information
dialog causes this crash:

======================
libparted : 2.4
======================
**
ERROR:Win_GParted.cc:989:void GParted::Win_GParted::set_valid_operations(): assertion failed: (valid_display_partition_ptr( selected_partition_ptr ))
Aborted (core dumped)

At this point in the code:

    973  void Win_GParted::set_valid_operations()
    974  {
    ...
    986          // No partition selected ...
    987          if ( ! selected_partition_ptr )
    988                  return ;
>>  989          g_assert( valid_display_partition_ptr( selected_partition_ptr ) );  // Bug: Not pointing at a valid display partition object

Bug 750168 - Reduce the amount of copying of partition objects