FileBase: Fix unsetting of default when destroyed

Code that should unset a FileBase from being the default when it is
destroyed was broken by a `==` instead of `=` typo.

platform/FileBase.cpp

@@ -57,7 +57,7 @@ FileBase::~FileBase()
     }
 
     if (_default == this) {
-        _default == NULL;
+        _default = NULL;
     }
 
     _mutex->unlock();