This is technically a denial of service because the dbus-daemon will
run out of memory eventually, but it's a very slow and noisy one,
because all the rejected messages are also very likely to have
been logged to the system log.

Detected by AddressSanitizer.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: dbus/dbus#234
Reviewed-by: pwithnall