chan: After sending the GSM04.08 RR Release, reset the subscriber and wait

After we send the SACH DEACTIVATE the BTS will get back to us with a
Release Indication which will trigger the RF Channel Release handling. This
is why we can return here, but we need to put the subscriber reference to
make sure to not end in a infinite loop.

This and the previous change fix the USSD issue for me.

openbsc/src/chan_alloc.c

@@ -381,6 +381,11 @@ static void _lchan_handle_release(struct gsm_lchan *lchan)
 		++lchan->conn.use_count;
 		gsm48_send_rr_release(lchan);
 		--lchan->conn.use_count;
+
+		/* avoid reentrancy */
+		subscr_put(lchan->conn.subscr);
+		lchan->conn.subscr = NULL;
+		return;
 	}
 
 	/* spoofed? message */