[PATCH] Fix segfault in 'svnauthz' (missing command table sentinel)

Hi,

This patch is a follow-up to r1428209. It adds the command table sentinel
for 'svnauthz' tool (tools/server-side/svnauthz.c).

The absense of this sentinel can result in undefined behavior (most likely
segfault), e.g., during a "svnauthz help" call — we're iterating over the
commands until we find NULL.

Thanks and regards,
Evgeny Kotkov