fix test clang diags driver bufnr

although it is impossible that exist diags bufnr but not exist
diags driver bufnr, anyway, a defensive test

Author: justmao945

plugin/clang.vim

@@ -408,7 +408,7 @@ func! s:DiagnosticsWindowClose(when_bufwinleave)
   endif
 
   " is not leave from the driver buffer window
-  if a:when_bufwinleave && l:cbn != t:clang_diags_driver_bufnr
+  if a:when_bufwinleave && (!exists('t:clang_diags_driver_bufnr') || l:cbn != t:clang_diags_driver_bufnr)
     return
   end