pkg/envoy/xds: Assign value to ip variable only if it's used
pkg/envoy/xds: Assign value to ip variable only if it's used

`ip` variable is used only once, so there is no need of assigning
the value of IstioNodeToIP everytime.

Signed-off-by: Michal Rostecki <82ebf970c69340aebb770c3853be5fd96a3eb584@suse.com>

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