libmtp: fix udev rules in case there is no valid ~/.mtpz-data

They managed to output error messages to stdout, which mustn't
go there. In some cases the output is supposed to be a valid udev-rules
file.
Merge pull request #261 from v4hn/libmtp-fix-udev-rules

libmtp: fix udev rules in case there is no valid ~/.mtpz-data