SPU: Reverb input should be muted when SPU muted

Fixes pause menu in Bubsy 3D, Tuning Car Battle Spec R.

src/core/spu.cpp

@@ -2357,6 +2357,8 @@ void SPU::Execute(void* param, TickCount ticks, TickCount ticks_late)
       {
         left_sum = 0;
         right_sum = 0;
+        reverb_in_left = 0;
+        reverb_in_right = 0;
       }
 
       // Update noise once per frame.