Do debug check for closing of DB before Project destruction safely...

... It wasn't safe, on some platforms, because the sequence of destruction
of ProjectFileIO and ConnectionPtr is unspecified.  It is safer that
ConnectionPtr just checks itself, not that ProjectFileIO fetches it again.

src/DBConnection.cpp

@@ -298,7 +298,10 @@ int DBConnection::CheckpointHook(void *data, sqlite3 *db, const char *schema, in
    return SQLITE_OK;
 }
 
-ConnectionPtr::~ConnectionPtr() = default;
+ConnectionPtr::~ConnectionPtr()
+{
+   wxASSERT_MSG(mpConnection, wxT("Project file was not closed at shutdown"));
+}
 
 static const AudacityProject::AttachedObjects::RegisteredFactory
 sConnectionPtrKey{

src/ProjectFileIO.cpp

@@ -254,7 +254,6 @@ ProjectFileIO::ProjectFileIO(AudacityProject &project)
 
 ProjectFileIO::~ProjectFileIO()
 {
-   wxASSERT_MSG(!CurrConn(), wxT("Project file was not closed at shutdown"));
 }
 
 sqlite3 *ProjectFileIO::DB()