websock: fix handshake leak

websock: fix handshake leak

Missed in f69a8bde293.
Thanks Valgrind:

==955== 217 bytes in 1 blocks are definitely lost in loss record 275 of 321
==955==    at 0x483A965: realloc (vg_replace_malloc.c:785)
==955==    by 0x50B6839: __vasprintf_chk (in /usr/lib64/libc-2.28.so)
==955==    by 0x49AA05C: g_vasprintf (in /usr/lib64/libglib-2.0.so.0.5800.1)
==955==    by 0x4983440: g_strdup_vprintf (in /usr/lib64/libglib-2.0.so.0.5800.1)
==955==    by 0x126048: qio_channel_websock_handshake_send_res (channel-websock.c:162)
==955==    by 0x1266E6: qio_channel_websock_handshake_send_res_ok (channel-websock.c:362)
==955==    by 0x126D3E: qio_channel_websock_handshake_process (channel-websock.c:468)
==955==    by 0x126EF2: qio_channel_websock_handshake_read (channel-websock.c:511)
==955==    by 0x12715B: qio_channel_websock_handshake_io (channel-websock.c:571)
==955==    by 0x125027: qio_channel_fd_source_dispatch (channel-watch.c:84)
==955==    by 0x496326C: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.5800.1)
==955==    by 0x169EC3: glib_pollfds_poll (main-loop.c:215)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>