Fix crash when passing None to Filename constructor

dtool/src/dtoolutil/filename.h

@@ -60,10 +60,11 @@ public:
     F_pattern         = 0x40,
   };
 
+  INLINE Filename(const char *filename);
+
 PUBLISHED:
   INLINE Filename(const string &filename = "");
   INLINE Filename(const wstring &filename);
-  INLINE Filename(const char *filename);
   INLINE Filename(const Filename &copy);
   Filename(const Filename &dirname, const Filename &basename);
   INLINE ~Filename();