testsprite: zero initialize textures

LoadSprite destroys current textures before allocating new ones.
So make sure the previous (=first) textures are valid.

test/testsprite.c

@@ -502,7 +502,7 @@ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
 
     /* Create the windows, initialize the renderers, and load the textures */
     sprites =
-        (SDL_Texture **)SDL_malloc(state->num_windows * sizeof(*sprites));
+        (SDL_Texture **)SDL_calloc(state->num_windows, sizeof(*sprites));
     if (!sprites) {
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Out of memory!");
         return SDL_APP_FAILURE;