[pkg-horde] CVE-2007-1515: imp4/etch not vulnerable

[Florian Weimer]

* Gregory Colpart:

> You saw probably the answer of Nico Golde but I give you more
> details here: a patch[*] for this issue was applied in imp4
> 4.1.3-4 (version currently in etch). This patch is a backport of
> upstream security changes.

FWIW, I've updated the track to reflect this.  In such cases, a single
entry is sufficient.