proxy.c refactoring: remove always-NULL gc parameter

proxy.c refactoring: remove always-NULL gc parameter

get_proxy_authenticate() is called with a "gc" parameter which MUST
be NULL, otherwise string_alloc() will allocate memory in the gc_arena
while the caller expects the result to be durable and will do explicit
free() when no longer needed.  Remove gc parameter, call string_alloc()
with "NULL" so this is clearly visible.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20170609205029.11002-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14768.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>