Minor tweak fixing 'cant use an undefined value as an ARRAY reference' error

The script has worked greaton most servers I've tried it on, however one of my systems with a hot-spare failure on a software/mdstat raid   was producing this error when i'd try to run the script:
[root@server nagios-plugin-check_raid]# ./check_raid.pl
Can't use an undefined value as an ARRAY reference at ./check_raid.pl line 773.

after this patch:
[root@server nagios-plugin-check_raid]# ./check_raid.pl 
WARNING: mdstat:[md0(299.94 MiB raid1):UU, md127(1.74 TiB raid1):hot-spare failure::UU, md1(68.30 GiB raid1):UU]