Changes

s1ap_proxy: fix IEI path leak in handle_ies/4

Function handle_ies/3 prepends a new item to the IEI path and calls
function handle_ies/4.  The later was expected to remove that item
from the IEI path upon returning the new #proxy_state.

This was done correctly in the successful scenario, however the
error case was overlooked in 00f51fe.  As a result of this, whenever
handle_ie/3 returns an error, the IEI path is not properly cleaned
and stale IEIs remain in the #proxy_state.

Fix this by removing the head IEI in the same place where we push it.

Change-Id: I64629a215c05e8d577a8943ae872945d61b5107c
Fixes: 00f51fe ("s1ap_proxy: handle_ies(): pass IEI path to handle_ie()")
Related: SYS#7288