zebra: Fix use-after-free issue in srte cleanup

zebra: Fix use-after-free issue in srte cleanup

Currently, in `zebra_srte_client_close_cleanup` we use the `RB_FOREACH`
macro to traverse the SR policies tree. We remove the SR policies within
the loop. Removing elements from the tree and freeing them is not safe
and causes a use-after-free crash whenever the
`zebra_srte_client_close_cleanup` is called to perform cleanup.

This commit replaces the `RB_FOREACH` macro with its variant
`RB_FOREACH_SAFE`. Unlike `RB_FOREACH`, `RB_FOREACH_SAFE` permits both
the removal of tree elements as well as freeing them from within the
loop safely.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>