The GByteArray allocated in the beginning is not freed in case
of error.

g_byte_array_append() can reallocate its data, so make sure that we
don't rely on any pointer pointing to it after calling it.

Also remove a spurious unref in the caller while handling the error,
which shouldn't be needed as the extension returns NULL in case of
error.

In case of error while processing an incoming or outgoing message, an
extension should return NULL as is customary. This way, in case of
error, the caller doesn't need to worry about having to deal with
dropping the reference for the returned value if non-NULL.

This was supported in the old Autotools build system, but was lost
in the move to Meson.

Signed-off-by: Simon McVittie <smcv@debian.org>
Fixes: https://gitlab.gnome.org/GNOME/libsoup/issues/163

We are currently ignoring the Set-Cookie in handshake response because
the SoupCookieJar feature handles the header in the got-headers signal
that is never emitted for informational messages. SoupCookieJar should
also handle Set-Cookie for switching protocols informational messages.