ftmod_isdn: Fix per-channel teletone buffer offset wraparound.

Do a "soft" wraparound with modulo, removes the ~0.5s tone glitch.
(Multiply ts.rate (samples) by two to match the offset unit (bytes, 2 per sample).)
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

libs/freetdm/src/ftmod/ftmod_isdn/ftmod_isdn.c

@@ -1964,9 +1964,7 @@ static void *ftdm_isdn_tones_run(ftdm_thread_t *me, void *obj)
 					data->offset += rlen;
 				}
 
-				if (data->offset >= ts.rate) {
-					data->offset = 0;
-				}
+				data->offset %= (ts.rate << 1);
 			}
 		}