Bug#845405: x11-common: configure fails at "db_purge" with exit status 20 "Incorrect number of arguments"

[Tianon Gravi]

> After today's upload, the configure step of "x11-common" fails
> (presumably thanks to the addition of "set -x"):

Errr, meant "set -e"; specifically the following (from the changelog):

>   * debian/x11-common.postinst: call `set -e'.

♥,
- Tianon
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