Changes

ipaccess: Reset pointer before calling stream_cli func triggering callback

The osmo_stream_cli_destroy() may trigger the disconnect_cb, which we
forward to the user through sign_link_down() callback.
Hence, the user may call e1inp_ipa_bts_rsl_close_n() again (re-entrant,
recursively) before we had time to nullify the pointer, calling
osmo_stream_cli_destroy() again.
Instead, make sure the pointer is reset bfore potentially calling the
osmo_stream_cli_destroy() so make sure to catch this scenario.

This issue was being triggered by BTS_Tests.TC_chopped_ipa_payload,
which was hitting an assert due to incorrectly calling
osmo_stream_cli_destroy() twice on a given stream_cli.

Related: SYS#6729
Change-Id: I2f1da24580effab5e3dfeacb6514c3b1e3a650c9