bug#23158: 25.1.50; crash by consecutive http accesses

[Katsumi Yamaoka]

(Recipe, bt full, and xbacktrace are below)

Hi,

I got a spam mail that crashes Emacs (at least the one built from
the git master and 24.5 bundled with Cygwin).  The mail contains
a lot of mail addresses in the From: header like this:

From: user<at>greenstream.in
        
ly.com,abehera<at>jmi.ac.in,Alana<at>jmpco.com,ahron<at>jmpeacecorps.gov,kyu<at>jmtek.com,campfijf<at>jmu.edu,cnrrao<at>jncasr.ac.in,jeffv<at>jndtho
        
mas.com,calpolyslofellow<at>jnf.org,edwin.rainbow<at>jnled.nl,a728399<at>jnxjn.com,cf<at>job114.cc,jroberts<at>jobeshenderson.com,salishnavaja
        ...

When trying to read it using Gnus, Gnus tries to fetch gravatar
image for each address, and those consecutive http accesses seem
to crash Emacs.  Here is a simplified code for `emacs -Q' to
reproduce the problem:

(progn
  (require 'gnus-gravatar)
  (require 'url)
  (with-temp-buffer
    (let ((url-cache-expire-time 0)
          (gnus-article-buffer (current-buffer)))
      (insert "From:")
      (dotimes (i 255) (insert " foo@example.com.invalid,"))
      (insert "\n")
      (gnus-gravatar-transform-address "from" 'from-gravatar))))

`bt full' and xbacktrace are attached.  Thanks in advance.

In GNU Emacs 25.1.50.1 (i686-pc-cygwin, GTK+ Version 3.14.13)
 of 2016-03-30 built on localhost
Windowing system distributor 'The Cygwin/X Project', version 11.0.11702000
Configured using:
 'configure --verbose --with-x-toolkit=gtk3 --with-xwidgets'

bt-full.gz
Description: application/gunzip

xbacktrace.gz
Description: application/gunzip