Try fixing a bug where a sound is never unmarked as mixing.

This should fix a deadlock when uninitializing a sound after it's
reached the end.

research/miniaudio_engine.h

@@ -8338,11 +8338,7 @@ static void ma_engine_mix_sound(ma_engine* pEngine, ma_sound_group* pGroup, ma_s
             ma_uint64 framesProcessed;
 
             /* If we're marked at the end we need to stop the sound and do nothing. */
-            if (pSound->atEnd) {
-                ma_sound_stop_internal(pSound);
-                return;
-            }
-
+            if (!pSound->atEnd) {
                 /* If we're seeking, do so now before reading. */
                 if (pSound->seekTarget != MA_SEEK_TARGET_NONE) {
                     pSound->seekTarget  = MA_SEEK_TARGET_NONE;
@@ -8405,6 +8401,10 @@ static void ma_engine_mix_sound(ma_engine* pEngine, ma_sound_group* pGroup, ma_s
                         ma_sound_stop_internal(pSound);
                     }
                 }
+            } else {
+                /* The sound is at the end. Make sure it's marked as stopped. */
+                ma_sound_stop_internal(pSound);
+            }
         }
     }
     c89atomic_exchange_32(&pSound->isMixing, MA_FALSE);