realloc in buf_append

[Mike Miller]

A possible memory leak in the buf_append function was just reported
[1]. At a glance it looks like most of the functions in http.c that
use buf_append/buf_error will return ENOMEM back up the stack and
openconnect will exit immediately. Users of the library should
presumably do the same. I think it's still worth fixing to be safe,
but do you see any possible conditions where this could be a problem?
Thanks.

[1] http://bugs.debian.org/700805

-- 
mike