MINOR: config: Don't dump keywords if argument is NULL

Helper functions are used to dump bind, server or filter keywords. These
functions are used to report errors during the configuration parsing. To have a
coherent API, these functions are now prepared to handle a null pointer as
argument. If so, no action is performed and functions immediately return.

This patch should fix the issue #631. It is not a bug. There is no reason to
backport it.
3 files changed