player_thread: wait for decoder before seeking

When the decoder initialization has not been completed yet, all calls
to dc_seek() will fail, because dc.seekable is not initialized yet.
Wait for the decoder to complete its initialization, i.e. until it has
called decoder_initialized().

src/decoder_control.c

@@ -86,6 +86,7 @@ dc_stop(struct notify *notify)
 bool
 dc_seek(struct notify *notify, double where)
 {
+	assert(dc.state != DECODE_STATE_START);
 	assert(where >= 0.0);
 
 	if (dc.state == DECODE_STATE_STOP ||