Fix bash completion and its helper

Koki Fukuda requested to merge kofuk/dbus-glib:fix-bash-completion into master

Current completion for dbus-send fails to complete word, and says error. This merge request fixes error in completion script causing this error.

Furthermore, the helper program SEGVs if completing bus is inaccessible. This MR also fixes this issue.
(You would be able to reproduce this issue by completing org.bluez.mesh service in system bus on a system with offline Bluetooth.)

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