[FixRM #8525] undefined method `+' for nil:NilClass in enum_ie

Looks like for some reason if CryptUnprotectData fails, the decrypt_reg() method will return "". And when you unpack "", you produce an array of nils. Since you cannot add something to nil, this should cause an
"undefined method `+' for nil:NilClass" error.

This will check if we get an array of nils, we jump to the next iteration.

See RM 8525:
http://dev.metasploit.com/redmine/issues/8525