Fix corner case in NTLM authentication (trac #172)

The problem is located in the file proxy.c within
"establish_http_proxy_passthru": To keep buffers small long
base64-encoded NTLM-Strings are truncated.

But the truncating is done on a wrong place: base 64 strings can be
cut every 4 chars. the buffer is 128 bytes - including the terminating
\0, so the usable data is only 127 bytes. And decoding a 127 char
base64 string fails... this is why the ntlm authentication fails in
certain cases (long strings)...

Acked-by: Joerg Willmann <joe@clnt.de>
URL: https://community.openvpn.net/openvpn/ticket/172

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f8ac53b98ed2513f1d80363b6fd2351f1b4ae511)