lbmap: Avoid panic in case backendsByMapIndex contains holes
lbmap: Avoid panic in case backendsByMapIndex contains holes

This should never happen but it may have happened. It's still unclear whether
this has happened due to a locking bug or another corruption. As the previous
commit has fixed locking, print a fat error when a hole is encountered but
avoid the panic.

Fixes: #6537

Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

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