Set the current buffer in esh-mode before running filter functions

Set the current buffer in esh-mode before running filter functions

* lisp/eshell/esh-mode.el: (eshell-output-filter): Match
current-buffer behavior of comint-output-filter (bug#42870).

This change (a) sets the current buffer to the process-buffer when
invoking preoutput filter functions and (b) only invokes them when the
process-buffer is live. Otherwise, the preoutput filter functions be
invoked in whatever buffer happens to be focused, breaking hooks that
read buffer-local variables.