bug#18116: 24.3.92; url-http calls CALLBACK recursively with malformed CBARGS if the former calls `delete-process'

[Dmitry <dgutov@yandex.ru>]

Note: this could be considered a regression from Emacs 24.3

With this test case:

```
(require 'url-http)

(defvar uht-counter 0)

(defun uht-callback (status)
  (declare (special url-http-process))
  (message "%s %s" uht-counter status)
  (delete-process url-http-process))

(defun uht-test ()
  (setq uht-counter (1+ uht-counter))
  ;; The port must not be open.
  (url-http (url-generic-parse-url "http://localhost:3333") #'uht-callback (list 'foo)))
```

Evaluate `(uht-test)' and see two messages in the message log with the
same counter value. The second message outputs a different STATUS value,
caused by modification of `url-callback-arguments' in
`url-http-async-sentinel', like this:

4 (:error (error connection-failed failed with code 111
 :host localhost :service 3333) . foo)
4 (:error (error connection-failed deleted
 :host localhost :service 3333) :error (error connection-failed failed with code 111
 :host localhost :service 3333) . foo)

If the callback expects STATUS to contain some specific data structure,
that can cause breakage, see https://github.com/marijnh/tern/issues/350
for an example.

Now, I'm not entirely sure the problem is in `url-http'. It does not
manifest in Emacs 24.3, possibly because in the current pretest, when
the connection fails, the `url-http-process' is still alive when
`delete-process' is called in `uht-callback'. This is not the case in
the current stable Emacs (wherein, as a consequence, `uht-callback'
doesn't get called the second time), so this could be a regression in
the processes subsystem.


In GNU Emacs 24.3.92.3 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2014-07-24 on axl
Repository revision: 117398 stephen.berman@gmx.net-20140722213204-51v7bp0chfei6wbx
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:	Ubuntu 14.04.1 LTS