Landing rapid7#1805 - Fix unintelligible error when importing empty file

Author: wchen-r7

lib/msf/core/db.rb

@@ -2797,6 +2797,9 @@ def import_file(args={}, &block)
 		::File.open(filename, 'rb') do |f|
 			data = f.read(4)
 		end
+		if data.nil?
+			raise DBImportError.new("Zero-length file")
+		end
 
 		case data[0,4]
 		when "PK\x03\x04"