bug#12145: Fix causes problems with other processes

[Thomas Koch]

The problem identified by the first backtrace still exists even with the patch 
for this bug from 2012. It happens, when find-name-dired on a remote system 
returns very fast (or maybe when the output is very short?), e.g. when called 
on an empty directory.

The function `find-dired` starts a background shell process:

    (shell-command (concat args "&") (current-buffer))

A few lines later, the function asks for the process and uses it without any 
error checking:

    (let ((proc (get-buffer-process (current-buffer))))
      (set-process-filter proc (function find-dired-filter))

I assume, that get-buffer-process returns nil when the process has already 
terminated.

I don't have an idea about the second backtrace yet. But given that the code of 
find-name-dired seems problematic, this might be a motivation to not search for 
a fix in tramp but in find-name-dired (and called functions).