Add an assert to fix a Coverity warning in SDL Sound

Part of issue #2996

Coverity is reporting a use after free bug in Sound_Quit.
It doesn't look like an actual bug since the code only removes elements
from the head of the linked list.
Add an assert so hopefully Coverity sees that we never take the "broken"
branch.

src/libs/decoders/SDL_sound.c

@@ -570,6 +570,11 @@ void Sound_FreeSample(Sound_Sample *sample)
     /* update the sample_list... */
     if (internal->prev != NULL)
     {
+        // We're not removing from the head.
+        // Assert that we're not touching sample_list (which should be the head of the linked list).
+        // This assert fixes a Coverity warning.
+        assert(sample_list != sample);
+
         Sound_SampleInternal *prevInternal;
         prevInternal = (Sound_SampleInternal *) internal->prev->opaque;
         prevInternal->next = internal->next;