api: Fix PatchEndpointIDConfig with unknown EPID
api: Fix PatchEndpointIDConfig with unknown EPID

This API previously didn't check whether the endpoint was non-nil
before attempting to use it, which would cause the API handler to panic.
Instead, check whether the endpoint exists and return an appropriate
error.

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

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