"undefined variable" bugs corrected

I added a @ before some references.

Appears when:

1/ Creating backup without choosing any of the three choices
Notice: Undefined index: exp-format in (...)/admin/maintenance.php on line 522
Notice: Undefined index: exp-format in (...)/admin/maintenance.php on line 550
Notice: Undefined index: exp-format in (...)/admin/maintenance.php on line 556

2/ Creating backup with a zip file (I'm running mysql)
Notice: Undefined index: incl-sqlit in (...)/admin/maintenance.php on line 558