better handling of hosts with no USB History

git-svn-id: file:///home/svn/framework3/trunk@14153 4d416f70-5f16-0410-b530-b9f4589650da

modules/post/windows/gather/usb_history.rb

@@ -51,8 +51,7 @@ class Metasploit3 < Msf::Post
 
 		usb_drive_classes = enum_subkeys('HKLM\\SYSTEM\\CurrentControlSet\\Enum\\USBSTOR')
 		usb_uids_to_info = {}
-		usb_drive_uids = []
-
+		if not usb_drive_classes.nil?
 			usb_drive_classes.each do |x|
 				enum_subkeys(x).each do |y|
 					begin
@@ -93,7 +92,9 @@ class Metasploit3 < Msf::Post
 				end
 				print_status(info_hash_to_str(out, v))
 			end
-
+		else
+			print_error("No USB devices appear to have been connected to theis host.")
+		end
 	end
 
 	#-------------------------------------------------------------------------------