Fix buffer-mismatch bug in erc-scroll-to-bottom

Fix buffer-mismatch bug in erc-scroll-to-bottom

* lisp/erc/erc-goodies.el (erc-scroll-to-bottom): Only `recenter' when
the selected window's buffer is current.  Previously, the module
`scrolltobottom' signaled an "Error in `post-command-hook'" when a
user clicked a channel indicator in the mode line from a window
showing another ERC buffer.
* lisp/erc/erc-track.el (erc-track--switch-fallback-blockers): New
internal variable used by `erc-track--switch-buffer' in deciding
whether to set `erc-track-last-non-erc-buffer' to the current buffer.
(erc-track--switch-buffer): Consult list of `buffer-match-p'
conditions in `erc-track--switch-fallback-blockers' to decide whether
to set `erc-track-last-non-erc-buffer' to the current buffer.
(Bug#63595)