Fix a stack trace when the SMBUser isn't set

For some reason an invalid user/pass don't seem to trigger
STATUS_ACCESS_DENIED responses, but an empty user does.

modules/auxiliary/scanner/smb/smb_enumshares.rb

@@ -108,7 +108,12 @@ class Metasploit3 < Msf::Auxiliary
 
 		simple.connect("IPC$")
 		handle = dcerpc_handle('4b324fc8-1670-01d3-1278-5a47bf6ee188', '3.0', 'ncacn_np', ["\\srvsvc"])
+		begin
 			dcerpc_bind(handle)
+		rescue Rex::Proto::SMB::Exceptions::ErrorCode => e
+			print_error("#{rhost} : #{e.message}")
+			return
+		end
 
 		stubdata =
 			NDR.uwstring("\\\\#{rhost}") +