Raise IOError when reading a blank file
Raise IOError when reading a blank file

Previously, a blank file passed to CFPropertyList::List.new() would
raise a NoMethodError on the magic_number variable as it would try
to address a range of characters with [].

This commit will raise an IOError in the event that magic_number is
not populated, and present a better message than a NoMethodError.

ckruse/CFPropertyList