fallback: fix crash when boot entry index >= size of BootOrder list
fallback: fix crash when boot entry index >= size of BootOrder list

Seen on Dell PowerEdge R740 when booting with BOOTX64 constantly.

This patch keeps the behaviour previous to commit #1b30c2b by returning
the index of the "Linux" entry.
Then a check is made to find the entry in the current BootOrder:
- if it isn't there, prepend the entry and copy the rest (this enlarges
  the BootOrder array by 1)
- if it's there, prepend the entry and copy all remaining entries

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Example of outputs on a Dell PowerEdge R740:
- 0000 is BOOTX64 entry
- other entries are Dell's default ones (internal, no "Linux" there)

1. Entry not already existing in BootOrder

----
set_boot_order:486: Original nbootorder: 3
Original BootOrder: 0000 0003 0004
 :
add_to_boot_list:578: device path: "HD(1,GPT,99D47E76-590F-48FD-8FD6-0A0CE790D635)/\EFI\redhat\shimx64.efi"
find_boot_option:454: Found boot entry "Boot0005" with label "Red Hat Enterprise Linux" for file "\EFI\redhat\shimx64.efi"
add_to_boot_list:623: New nbootorder: 4
BootOrder: 0005 0000 0003 0004
find_boot_options:937: Found directory named "Dell"
update_boot_order:509: nbootorder: 4
BootOrder: 0005 0000 0003 0004
----

2. Entry not existing at all

----
set_boot_order:486: Original nbootorder: 3
Original BootOrder: 0000 0001 0002
 :
add_to_boot_list:578: device path: "HD(1,GPT,99D47E76-590F-48FD-8FD6-0A0CE790D635)/\EFI\redhat\shimx64.efi"
add_boot_option:245: Creating boot entry "Boot0005" with label "Red Hat Enterprise Linux" for file "\EFI\redhat\shimx64.efi"
add_boot_option:282: nbootorder: 4
BootOrder: 0005 0000 0001 0002
find_boot_options:937: Found directory named "Dell"
update_boot_order:509: nbootorder: 4
BootOrder: 0005 0000 0001 0002
----

3. Entry already existing in BootOrder

----
set_boot_order:486: Original nbootorder: 4
Original BootOrder: 0000 0005 0001 0002
 :
add_to_boot_list:578: device path: "HD(1,GPT,99D47E76-590F-48FD-8FD6-0A0CE790D635)/\EFI\redhat\shimx64.efi"
find_boot_option:454: Found boot entry "Boot0005" with label "Red Hat Enterprise Linux" for file "\EFI\redhat\shimx64.efi"
add_to_boot_list:623: New nbootorder: 4
BootOrder: 0005 0000 0001 0002
find_boot_options:937: Found directory named "Dell"
update_boot_order:509: nbootorder: 4
BootOrder: 0005 0000 0001 0002
----

rhinstaller/shim,rhinstaller/shim,rhinstaller/shim