add correct return value vor Swift callback

deltachat-ios/wrapper.c

@@ -13,6 +13,5 @@ long callbackSwift(int, long, long, const char*, const char*);
 
  uintptr_t callback_ios(mrmailbox_t* mailbox, int event, uintptr_t data1, uintptr_t data2)
 {
-    callbackSwift(event, data1, data2, (const char*)data1, (const char*)data2);
-    return 0;
+    return callbackSwift(event, data1, data2, (const char*)data1, (const char*)data2);
 }