ipsec: fix edge case detection in key_do_getnewspi

The 'count' variable would end up being -1 post loop, while the
following condition would check for 0 instead.

PR: 258849
Reported by: Herbie.Robinson@stratus.com
Reviewed by: ae
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32826