Fix crash in Create New Partition dialog when changing type (#101)

On an MSDOS partitioned drive, open the Create New Partition dialog and
change "created as" from Primary Partition to Extended Partition and
back to Primary Partition.  On Fedora and RHEL/CentOS 8, which builds
packages with FORTIFY_SOURCE [1][2] and GLIBXX_Assertions [3][4]
enabled, GParted will crash.

Run GParted built with the default compilation options under valgrind
and repeat the test.  Multiple out of bounds reads are reported like
this:
  # valgrind --track-origins=yes ./gpartedbin
  ...
  ==232613== Invalid read of size 8
  ==232613==    at 0x441AF6: GParted::Dialog_Partition_New::combobox_changed(bool) (Dialog_Partition_New.cc:354)
  ==232613==    by 0x443DBD: sigc::bound_mem_functor1<void, GParted::Dialog_Partition_New, bool>::operator()(bool const&) const (mem_fun.h:2066)

Coming from Dialog_Partition_New.cc:
  328  void Dialog_Partition_New::combobox_changed(bool type)
  329  {
  ...
  351      // combo_filesystem and combo_alignment
  352      if ( ! type )
  353      {
> 354          fs = FILESYSTEMS[combo_filesystem.get_active_row_number()];

When the partition type is changed to Extended the file system is forced
to be "Extended" too.  This is done in ::combobox_changed() method by
modifying combo_filesystem to add "Extended", making that the selected
item and setting the widget as inactive.

Then when the partition type is changed back to primary the file system
combobox is returned to it's previous state.  This is done by first
removing the last "Extended" item, making the widget active and setting
the selected item.  However as "Extended" is the currently selected
item, removing it forces their to be no selected item and triggers a
change to combo_filesystem triggering a recursive call to
::combobox_changed() where combo_filesystem.get_active_row_number()
returns -1 (no selection) [5] and on line 354 the code accesses item -1
of the FILESYSTEMS[] vector.

Fix by setting the new combo_filesystem selection before removing the
currently selected "Extended" item.  This has the added benefit of only
triggering a change to combo_filesystem once when the default item is
selected rather than twice when the currently "Extended" item is removed
and again when the default item is selected.

[1] [Fedora] Security Features, Compile Time Buffer Checks
    (FORTIFY_SOURCE)
    https://fedoraproject.org/wiki/Security_Features#Compile_Time_Buffer_Checks_.28FORTIFY_SOURCE.29

[2] Enhance application security with FORTIFY_SOURCE
    https://access.redhat.com/blogs/766093/posts/1976213

[3] Security Features Matrix (GLIBXX_Assertions)
    https://fedoraproject.org/wiki/Security_Features_Matrix

[4] GParted 1.2.0-1.fc33 package build.log for Fedora 33
    https://kojipkgs.fedoraproject.org/packages/gparted/1.2.0/1.fc33/data/logs/x86_64/build.log
    CXXFLAGS='-O2 -g ... -Wp,-D_FORTIFY_SOURCE=2
    -Wp,-D_GLIBCXX_ASSERTIONS ...'

[5] gtkmm: Gtk::ComboBox Class Reference, get_active_row_number()
    https://developer.gnome.org/gtkmm/stable/classGtk_1_1ComboBox.html#a53531bc041b5a460826babb8496c363b

Closes #101 - Crash changing Partition type in "Create new partition"
              dialog

src/Dialog_Partition_New.cc

@@ -342,9 +342,9 @@ void Dialog_Partition_New::combobox_changed(bool type)
 		else if (combo_type.get_active_row_number() != TYPE_EXTENDED      &&
 		         combo_filesystem.items().size()    == FILESYSTEMS.size()   )
 		{
+			combo_filesystem.set_active(first_creatable_fs);
 			combo_filesystem.items().erase(combo_filesystem.items().back());
 			combo_filesystem.set_sensitive(true);
-			combo_filesystem.set_active(first_creatable_fs);
 		}
 	}