bgpd: fix error response to invalid BGP version number
BGP4-ANVL 20.1 ANVL tries to open BGP with version 5 and expects correct
notification in response. Quagga sends notification, but with incorrect
information in it.
The data needs to be a 2-byte value, and for now we respond with 0004 for any
peer version other than 4.
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
