Document that you can pass NULL to SDL_PeepEvents()

Fixes https://github.com/libsdl-org/sdlwiki/issues/547

src/events/SDL_events.c

@@ -878,6 +878,10 @@ static int SDL_PeepEventsInternal(SDL_Event *events, int numevents, SDL_EventAct
             return -1;
         }
         if (action == SDL_ADDEVENT) {
+            if (!events) {
+                SDL_UnlockMutex(SDL_EventQ.lock);
+                return SDL_InvalidParamError("events");
+            }
             for (i = 0; i < numevents; ++i) {
                 used += SDL_AddEvent(&events[i]);
             }