fixing grub2 faillure resulting in draklive-install crash (mga#16246)

thus fixing mga#15689 for live installs

The check for whether an ESP needs to be created or not seems to prevent
us to suggest a mount point for the existing one in some cases

to be reviewed for mga6...

diff --git a/perl-install/fs/partitioning_wizard.pm b/perl-install/fs/partitioning_wizard.pm
index f3b9973..b222c07 100644
--- a/perl-install/fs/partitioning_wizard.pm

create_display_box gets called twice. The first time, only the $kind
parameter has a value, the second time all the parameters (including
$button) have values.

On the first time through the partitioning step, the second call to
create_display_box sets the selected_for_resize flag for my NTFS
partition. When returning to the partitioning step, this flag is still
set, so the first call to create_display_box executes the block of code
that ends with

  $button->signal_connect('focus-in-event' => sub { ... });

even though it doesn't have a value for $button.
(Martin Whitaker, mga#15919)