Added Invalid Request Drop to AchievementManager Request

This fixes a crash I found in the Request function of AchievementManager where under certain conditions init_request would return an api_request with null post data, and Post would crash if it attempted to access it. Now the function aborts before the Post and returns an INVALID_REQUEST response type.

Source/Core/Core/AchievementManager.cpp

@@ -384,6 +384,8 @@ AchievementManager::ResponseType AchievementManager::Request(
   rc_api_request_t api_request;
   Common::HttpRequest http_request;
   init_request(&api_request, &rc_request);
+  if (!api_request.post_data)
+    return ResponseType::INVALID_REQUEST;
   auto http_response = http_request.Post(api_request.url, api_request.post_data);
   rc_api_destroy_request(&api_request);
   if (http_response.has_value() && http_response->size() > 0)

Source/Core/Core/AchievementManager.h

@@ -26,6 +26,7 @@ public:
   {
     SUCCESS,
     MANAGER_NOT_INITIALIZED,
+    INVALID_REQUEST,
     INVALID_CREDENTIALS,
     CONNECTION_FAILED,
     UNKNOWN_FAILURE