Changes

{erab_fsm,sctp_proxy}: handle_event/4: fix return value

When we do not reply, it must be an atom alone, not a tuple:

https://www.erlang.org/doc/apps/stdlib/gen_statem#t:state_callback_result/2

This patch fixes errors like this:

  State machine <0.406.0> terminating.
  Reason: {bad_return_from_state_function,{keep_state_and_data}}

Change-Id: Ib8c10e0b1d888e005a593644c0ecead560b2f2ce
Related: SYS#7318