Fix commands `reopen-last-buffer` and `reopen-buffer`

[André A. Gomes]

PR #3325 broke these commands and it is easy to fix it.

Additionally, prior to #3325 (i.e. in the current 3.11.1 release), there is another bug related to these commands.

Steps to reproduce:

• (nyxt:start :failsafe t)
• make-buffer-focus, navigate to URL and kill it
• reopen-last-buffer
• switch-buffer => the recovered buffer isn't listed

This is because the recovery doesn't re-populate the hash table buffers slot of class browser.