emacs: Fix "no such file or directory" error

Occasionally, when the user killed the search buffer when the CLI
process was still running, Emacs would run the
notmuch-start-notmuch-sentinel sentinel twice.  The first call would
process and delete the error output file and the second would fail
with an "Opening input file: no such file or directory, ..." error
when attempting to access the error file.

Emacs isn't supposed to run the sentinel twice.  The reason it does is
rather subtle (and probably a bug in Emacs):

1) When the user kills the search buffer, Emacs invokes
kill_buffer_processes, which sends a SIGHUP to notmuch, but doesn't do
anything else.  Meanwhile, suppose the notmuch search process has
printed some more output, but Emacs hasn't consumed it yet (this is
critical and is why this error only happens sometimes).

2) Emacs gets a SIGCHLD from the dying notmuch process, which invokes
handle_child_signal, which sets the new process status, but can't do
anything else because it's a signal handler.

3) Emacs returns to its idle loop, which calls status_notify, which
sees that the notmuch process has a new status.  This is where things
get interesting.

3.1) Emacs guarantees that it will run process filters on any
unconsumed output before running the process sentinel, so
status_notify calls read_process_output, which consumes the final
output and calls notmuch-search-process-filter.

3.1.1) notmuch-search-process-filter checks if the search buffer is
still alive and, since it's not, it calls delete-process.

3.1.1.1) delete-process correctly sees that the process is already
dead and doesn't try to send another signal, *but* it still modifies
the status to "killed".  To deal with the new status, it calls
status_notify.  Dun dun dun.  We've seen this function before.

3.1.1.1.1) The *recursive* status_notify invocation sees that the
process has a new status and doesn't have any more output to consume,
so it invokes our sentinel and returns.

3.2) The outer status_notify call (which we're still in) is now done
flushing pending process output, so it *also* invokes our sentinel.

This patch addresses this problem at step 3.1.1, where the filter
calls delete-process, since this is a strange and redundant thing to
do anyway.

emacs/notmuch.el

@@ -821,8 +821,7 @@ non-authors is found, assume that all of the authors match."
 	(parse-buf (process-get proc 'parse-buf))
 	(inhibit-read-only t)
 	done)
-    (if (not (buffer-live-p results-buf))
-	(delete-process proc)
+    (when (buffer-live-p results-buf)
       (with-current-buffer parse-buf
 	;; Insert new data
 	(save-excursion