Don't try relative paths in wxFileSystem if current path is empty

This seems redundant, as the loop is the same as the one just below it
which is executed in any case if this one doesn't find anything.

src/common/filesys.cpp

@@ -478,7 +478,7 @@ wxFSFile* wxFileSystem::OpenFile(const wxString& location, int flags)
     m_LastName.clear();
 
     // try relative paths first :
-    if (meta != wxT(':'))
+    if (meta != wxT(':') && !m_Path.empty())
     {
         node = m_Handlers.GetFirst();
         while (node)