BUG/MINOR: stream: protect stream_dump() against incomplete streams

If a stream is interrupted during its initialization by a panic signal
and tries to dump itself, it may cause a crash during the dump due to
scf and/or scb not being fully initialized. This may also happen while
releasing an endpoint to attach a new one. The effect is that instead
of dying on an abort, the process dies on a segv. This race is ultra-
rare but totally possible. E.g:

  #0  se_fl_test (test=1, se=0x0) at include/haproxy/stconn.h:98
  #1  sc_ep_test (test=1, sc=0x7ff8d5cbd560) at include/haproxy/stconn.h:148
  haproxy#2  sc_conn (sc=0x7ff8d5cbd560) at include/haproxy/stconn.h:223
  haproxy#3  stream_dump (buf=buf@entry=0x7ff9507e7678, s=0x7ff4c40c8800, pfx=pfx@entry=0x55996c558cb3 ' ' <repeats 13 times>, eol=eol@entry=10 '\n') at src/stream.c:2840
  haproxy#4  0x000055996c493b42 in ha_task_dump (buf=buf@entry=0x7ff9507e7678, task=<optimized out>, pfx=pfx@entry=0x55996c558cb3 ' ' <repeats 13 times>) at src/debug.c:328
  haproxy#5  0x000055996c493edb in ha_thread_dump_one (thr=thr@entry=18, from_signal=from_signal@entry=0) at src/debug.c:227
  haproxy#6  0x000055996c493ff1 in ha_thread_dump (buf=buf@entry=0x7ff9507e7678, thr=thr@entry=18) at src/debug.c:270
  haproxy#7  0x000055996c494257 in ha_panic () at src/debug.c:430
  haproxy#8  ha_panic () at src/debug.c:411
  (...)
  haproxy#23 0x000055996c341fe8 in ssl_sock_close (conn=<optimized out>, xprt_ctx=0x7ff8dcae3880) at src/ssl_sock.c:6699
  haproxy#24 0x000055996c397648 in conn_xprt_close (conn=0x7ff8c297b0c0) at include/haproxy/connection.h:148
  haproxy#25 conn_full_close (conn=0x7ff8c297b0c0) at include/haproxy/connection.h:192
  haproxy#26 h1_release (h1c=0x7ff8c297b3c0) at src/mux_h1.c:1074
  haproxy#27 0x000055996c39c9f0 in h1_detach (sd=<optimized out>) at src/mux_h1.c:3502
  haproxy#28 0x000055996c474de4 in sc_detach_endp (scp=scp@entry=0x7ff9507e3148) at src/stconn.c:375
  haproxy#29 0x000055996c4752a5 in sc_reset_endp (sc=<optimized out>, sc@entry=0x7ff8d5cbd560) at src/stconn.c:475

Note that this cannot happen on "show sess" since a stream never leaves
process_stream in such an uninitialized state, thus it's really only the
crash dump that may cause this.

It should be backported to 2.8.

src/stream.c

@@ -2831,16 +2831,16 @@ void stream_dump(struct buffer *buf, const struct stream *s, const char *pfx, ch
 	res = &s->res;
 
 	scf = s->scf;
-	cof = sc_conn(scf);
-	acf = sc_appctx(scf);
+	cof = (scf && scf->sedesc) ? sc_conn(scf) : NULL;
+	acf = (scf && scf->sedesc) ? sc_appctx(scf) : NULL;
 	if (cof && cof->src && addr_to_str(cof->src, pn, sizeof(pn)) >= 0)
 		src = pn;
 	else if (acf)
 		src = acf->applet->name;
 
 	scb = s->scb;
-	cob = sc_conn(scb);
-	acb = sc_appctx(scb);
+	cob = (scb && scb->sedesc) ? sc_conn(scb) : NULL;
+	acb = (scb && scb->sedesc) ? sc_appctx(scb) : NULL;
 	srv = objt_server(s->target);
 	if (srv)
 		dst = srv->id;