use SDL_InvalidParamError or SDL_assert instead of custom SDL_SetError

src/audio/SDL_audiocvt.c

@@ -930,9 +930,7 @@ SDL_AddAudioCVTFilter(SDL_AudioCVT *cvt, const SDL_AudioFilter filter)
     if (cvt->filter_index >= SDL_AUDIOCVT_MAX_FILTERS) {
         return SDL_SetError("Too many filters needed for conversion, exceeded maximum of %d", SDL_AUDIOCVT_MAX_FILTERS);
     }
-    if (filter == NULL) {
-        return SDL_SetError("Audio filter pointer is NULL");
-    }
+    SDL_assert(filter != NULL);
     cvt->filters[cvt->filter_index++] = filter;
     cvt->filters[cvt->filter_index] = NULL; /* Moving terminator */
     return 0;