endpoint: refactor negative duration check
endpoint: refactor negative duration check

Replace "time.Since" with "safetime.TimeSinceSafe" which is guaranteed
to return a non-negative duration.

Signed-off-by: Martynas Pumputis <b14a8f953b021d09d1aafdca53cb27d7d9cede0c@covalent.io>

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