asprintf() returns -1 on error
asprintf() returns -1 on error

Fixes the bugs reported on the mailing list and Ubuntu tracker…

- http://lists.infradead.org/pipermail/openconnect-devel/2018-December/005164.html "Failed to obtain WebVPN cookie introduced by f08767d9644029bd3ac0e83bf160a7bf03a5c8de"
- https://bugs.launchpad.net/bugs/1809839 "[Bug 1809839] [NEW] Daily build 2667 broken for protocol=GP"

brandt/openconnect,brandt/openconnect,brandt/openconnect,brandt/openconnect