Fix harmless MSVC warning.

Avoid "converting value to bool" warning introduced by r75310.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75445 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/cmdproc.cpp

@@ -333,7 +333,7 @@ bool wxCommandProcessor::IsDirty() const
     {
         // We have never been saved, so we are dirty if and only if we have any
         // commands at all.
-        return m_currentCommand;
+        return m_currentCommand != NULL;
     }
 
     if ( !m_currentCommand )