net/miniupnpc: backport TALOS-2015-0035 (aka CVE-2015-6031) fix

Direct commit as /head in r399209 updated miniupnpc to a snapshot instead.

PR: 203705
Approved by: portmgr (bapt)
Security: 06fefd2f-728f-11e5-a371-14dae9d210b8
Differential Revision: https://reviews.freebsd.org/D3895