daemon: Check endpoint exists before PATCH
daemon: Check endpoint exists before PATCH

It appears that it's possible for endpoint PATCH and DELETE API calls to
arrive in quick succession, and for the PATCH to retrieve a reference to
an endpoint just before the DELETE removes it from the endpoint manager.

In this case, deleteEndpointQuiet() will remove the endpoint and
transition it to StateDisconnecting, then the PATCH will attempt to
revive the endpoint, however the newly resurrected endpoint will have no
controller and will be unable to determine its identity.

Related: #3268

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@covalent.io>

scanf/cilium,eloycoto/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,cilium-team/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,scanf/cilium,tklauser/cilium,scanf/cilium,scanf/cilium,eloycoto/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,eloycoto/cilium,scanf/cilium,eloycoto/cilium,tgraf/cilium,eloycoto/cilium,eloycoto/cilium,michi-covalent/cilium,scanf/cilium,scanf/cilium,cilium-team/cilium,eloycoto/cilium,tklauser/cilium,michi-covalent/cilium