opensl: Add extra asserts to help track down BMO 1913284.

src/cubeb_opensl.cpp

@@ -1857,6 +1857,9 @@ opensl_stream_get_position(cubeb_stream * stm, uint64_t * position)
   uint64_t samplerate = stm->user_output_rate;
   uint32_t output_latency = stm->output_latency_ms;
 
+  XASSERT(stm->written >= 0);
+  XASSERT(stm->user_output_rate > 0);
+  XASSERT(stm->output_configured_rate > 0);
   pthread_mutex_lock(&stm->mutex);
   int64_t maximum_position = stm->written * (int64_t)stm->user_output_rate /
                              stm->output_configured_rate;