Check for removal of scratch buffer

[Alex-ABPerson]

The plugin (and by extension the commandline itself, for the most part) gets bricked if the scratch buffer it uses is taken away. On my current nvim config, even the simple act of switching tabs does this. New check added to ensure the buffer is recreated if it does.

Before:

if not M.wbufnr then

After:

if not M.wbufnr or f.bufexists(M.wbufnr) == 0 then

Thanks!