MINOR: stream-int: only use si_release_endpoint() to release a connection

MINOR: stream-int: only use si_release_endpoint() to release a connection

Change si_alloc_conn() to call si_release_endpoint() instead of
open-coding the connection releasing code when reuse is disabled.
This fuses the code with the one already dealing with applets, makes
it shorter and helps centralizing the connection freeing logic at a
single place.