bug#29450: 26.0.90; No check for nil in some filenotify functions

[John Wiegley]

The documentation for find-file-name-handler says:

    find-file-name-handler is a built-in function in ‘C source code’.
    
    (find-file-name-handler FILENAME OPERATION)
    
    Return FILENAME’s handler function for OPERATION, if it has one.
    Otherwise, return nil.

However, several of the functions in filenotify use the return value of this
function without checking if it's nil or not:

    (defun file-notify-rm-watch (descriptor)
      "Remove an existing watch specified by its DESCRIPTOR.
    DESCRIPTOR should be an object returned by `file-notify-add-watch'."
      (when-let* ((watch (gethash descriptor file-notify-descriptors)))
        (let ((handler (find-file-name-handler
                        (file-notify--watch-directory watch)
                        'file-notify-rm-watch)))
          (condition-case nil
              (if handler
                  ;; A file name handler could exist even if there is no
                  ;; local file notification support.
                  (funcall handler 'file-notify-rm-watch descriptor)

I've been getting several errors with a backtrace like nil(48). This is likely
because some package has done something wrong, but even still, filenotify
should be more defensive.

-- 
John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2