bap: Fix not setting stream to NULL

If the stream state is idle the ep->stream shall be set to NULL
otherwise it may be reused causing the following trace:

==32623==ERROR: AddressSanitizer: heap-use-after-free on address ...
 READ of size 8 at 0x60b000103550 thread T0
    #0 0x7bf7b7 in bap_stream_valid src/shared/bap.c:4065
    ruundii#1 0x7bf981 in bt_bap_stream_config src/shared/bap.c:4082
    bluez#2 0x51a7c8 in bap_config profiles/audio/bap.c:584
    bluez#3 0x71b907 in queue_foreach src/shared/queue.c:207
    bluez#4 0x51b61f in select_cb profiles/audio/bap.c:626
    bluez#5 0x4691ed in pac_select_cb profiles/audio/media.c:884
    bluez#6 0x4657ea in endpoint_reply profiles/audio/media.c:369

Fixes: bluez#457 (comment)

profiles/audio/bap.c

@@ -998,9 +998,10 @@ static void bap_state(struct bt_bap_stream *stream, uint8_t old_state,
 	switch (new_state) {
 	case BT_BAP_STREAM_STATE_IDLE:
 		/* Release stream if idle */
-		if (ep)
+		if (ep) {
 			bap_io_close(ep);
-		else
+			ep->stream = NULL;
+		} else
 			queue_remove(data->streams, stream);
 		break;
 	case BT_BAP_STREAM_STATE_CONFIG: