BUILD: stream-int: avoid a build warning when DEBUG is empty

BUILD: stream-int: avoid a build warning when DEBUG is empty

When no DEBUG_STRICT is enabled, we get this build warning:

  src/stream_interface.c: In function 'stream_int_chk_snd_conn':
  src/stream_interface.c:1198:28: warning: unused variable 'conn' [-Wunused-variable]
   1198 |         struct connection *conn = cs_conn(cs);
        |                            ^~~~

This was the result of the simplification of the code in commit
d1480cc8a ("BUG/MEDIUM: stream-int: do not rely on the connection error
once established") which removed the last user of this variable outside
of a BUG_ON().

If the patch above is backported, this one should be backported as well.