hs_pow: fix insufficient length check in pow-params

The descriptor validation table had an out of date minimum length for pow-params (3) whereas the spec and the current code expect at least 4 parameters. This was an opportunity for a malicious service to cause an assert failure in clients which attempted to parse its descriptor.

Addresses issue #40793 (closed)

Signed-off-by: Micah Elizabeth Scott beth@torproject.org