[datagram] fix creating datagram without packet
[datagram] fix creating datagram without packet

When creating empty datagram add a single iovec with an empty buffer.
Also ensure the _parse stack is initialised.

Signed-off-by: Nikolay Nikolaev <5fb6ff14f2a3b931c007d120a649a55e86928f6a@virtualopensystems.com>

wingo/snabb,dpino/snabb,xdel/snabbswitch,lukego/snabbswitch,kbara/snabb,heryii/snabb,hb9cwp/snabbswitch,Igalia/snabb,dpino/snabb,pirate/snabbswitch,alexandergall/snabbswitch,dpino/snabbswitch,andywingo/snabbswitch,SnabbCo/snabbswitch,fhanik/snabbswitch,aperezdc/snabbswitch,aperezdc/snabbswitch,mixflowtech/logsensor,lukego/snabb,lukego/snabb,heryii/snabb,Igalia/snabb,eugeneia/snabb,SnabbCo/snabbswitch,eugeneia/snabb,pirate/snabbswitch,kbara/snabb,eugeneia/snabb,Igalia/snabbswitch,dpino/snabbswitch,snabbco/snabb,andywingo/snabbswitch,Igalia/snabb,javierguerragiraldez/snabbswitch,andywingo/snabbswitch,fhanik/snabbswitch,dpino/snabb,plajjan/snabbswitch,pavel-odintsov/snabbswitch,xdel/snabbswitch,alexandergall/snabbswitch,snabbnfv-goodies/snabbswitch,wingo/snabb,hb9cwp/snabbswitch,mixflowtech/logsensor,snabbco/snabb,virtualopensystems/snabbswitch,eugeneia/snabb,kellabyte/snabbswitch,Igalia/snabbswitch,lukego/snabb,dpino/snabb,eugeneia/snabbswitch,Igalia/snabb,Igalia/snabb,alexandergall/snabbswitch,justincormack/snabbswitch,dpino/snabbswitch,kbara/snabb,javierguerragiraldez/snabbswitch,andywingo/snabbswitch,heryii/snabb,snabbco/snabb,snabbco/snabb,justincormack/snabbswitch,lukego/snabbswitch,Igalia/snabbswitch,wingo/snabbswitch,snabbnfv-goodies/snabbswitch,snabbnfv-goodies/snabbswitch,eugeneia/snabbswitch,eugeneia/snabb,lukego/snabbswitch,mixflowtech/logsensor,SnabbCo/snabbswitch,dpino/snabb,alexandergall/snabbswitch,kbara/snabb,heryii/snabb,dpino/snabb,wingo/snabb,fhanik/snabbswitch,eugeneia/snabbswitch,SnabbCo/snabbswitch,kbara/snabb,plajjan/snabbswitch,Igalia/snabbswitch,eugeneia/snabbswitch,pavel-odintsov/snabbswitch,dpino/snabbswitch,wingo/snabbswitch,eugeneia/snabb,alexandergall/snabbswitch,mixflowtech/logsensor,Igalia/snabb,Igalia/snabb,kbara/snabb,eugeneia/snabb,snabbco/snabb,kellabyte/snabbswitch,hb9cwp/snabbswitch,alexandergall/snabbswitch,xdel/snabbswitch,virtualopensystems/snabbswitch,pirate/snabbswitch,Igalia/snabbswitch,justincormack/snabbswitch,lukego/snabb,wingo/snabbswitch,justincormack/snabbswitch,dwdm/snabbswitch,wingo/snabb,snabbco/snabb,aperezdc/snabbswitch,aperezdc/snabbswitch,heryii/snabb,virtualopensystems/snabbswitch,heryii/snabb,lukego/snabbswitch,eugeneia/snabb,hb9cwp/snabbswitch,kellabyte/snabbswitch,pavel-odintsov/snabbswitch,snabbco/snabb,dpino/snabb,snabbnfv-goodies/snabbswitch,plajjan/snabbswitch,mixflowtech/logsensor,wingo/snabb,plajjan/snabbswitch,javierguerragiraldez/snabbswitch,wingo/snabbswitch,alexandergall/snabbswitch,snabbco/snabb,wingo/snabb,dwdm/snabbswitch,Igalia/snabb,alexandergall/snabbswitch,dwdm/snabbswitch