http: Fix overflow on HTTP request buffers

http: Fix overflow on HTTP request buffers

A malicious VPN gateway can send a very long hostname/path (for redirects)
or cookie list (in general), which OpenConnect will attempt to sprintf()
into a fixed length buffer.  Each HTTP server response line can add
roughly MAX_BUF_LEN (131072) bytes to the next OpenConnect HTTP request,
but the request buffer (buf) is capped at MAX_BUF_LEN bytes and is
allocated on the stack.

The result of passing a long "Location:" header looks like:

    Attempting to connect to server 127.0.0.1:443
    SSL negotiation with localhost
    Server certificate verify failed: self signed certificate in certificate chain
    Connected to HTTPS on localhost
    GET https://localhost/
    Got HTTP response: HTTP/1.0 301 Moved
    Ignoring unknown HTTP response line 'aaaaaaaaaaaaaaaaaa'
    SSL negotiation with localhost
    Server certificate verify failed: self signed certificate in certificate chain
    Connected to HTTPS on localhost
    *** buffer overflow detected ***: /scr/openconnect2/.libs/lt-openconnect terminated
    ======= Backtrace: =========
    /lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x5c)[0x7fd62729b82c]
    /lib/x86_64-linux-gnu/libc.so.6(+0x109700)[0x7fd62729a700]
    /lib/x86_64-linux-gnu/libc.so.6(+0x108b69)[0x7fd627299b69]
    /lib/x86_64-linux-gnu/libc.so.6(_IO_default_xsputn+0xdd)[0x7fd62720d13d]
    /lib/x86_64-linux-gnu/libc.so.6(_IO_vfprintf+0x1ae7)[0x7fd6271db4a7]
    /lib/x86_64-linux-gnu/libc.so.6(__vsprintf_chk+0x94)[0x7fd627299c04]
    /lib/x86_64-linux-gnu/libc.so.6(__sprintf_chk+0x7d)[0x7fd627299b4d]
    /scr/openconnect2/.libs/libopenconnect.so.2(openconnect_obtain_cookie+0xc0)[0x7fd62832d210]
    /scr/openconnect2/.libs/lt-openconnect[0x40413f]
    /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xed)[0x7fd6271b276d]
    /scr/openconnect2/.libs/lt-openconnect[0x404579]

The proposed fix is to use dynamically allocated buffers with overflow
checking.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>