extensions: libebt_stp: fix range checking

extensions: libebt_stp: fix range checking

This has to either consider ->nvals > 1 or check the values
post-no-range-fixup:

./iptables-test.py  extensions/libebt_stp.t
extensions/libebt_stp.t: ERROR: line 12 (cannot load: ebtables -A INPUT --stp-root-cost 1)

(it tests 0 < 1 and fails, but test should be 1 < 1).

Fixes: dc6efcfeac38 ("extensions: libebt_stp: Use guided option parser")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>