bug#7841: 24.0.50; Freeze in imagemagick

[Stefan Monnier <monnier@iro.umontreal.ca>]

When I opened a PDF file (via doc-view), my Emacs froze.  I can
reproduce it at will, with apparently any pdf file:

  % emacs -Q foo.pdf
  [...wait for the first page to be displayed at then...]
  <page-down>

at which point Emacs freezes with the following message on stderr:

   emacs: unable to initialize module loader `Ressource temporairement non disponible'.

and the backtrace:

(gdb) bt
#0  0xb7fe2424 in __kernel_vsyscall ()
#1  0xb7707c39 in __lll_lock_wait () from /lib/i686/cmov/libpthread.so.0
#2  0xb770303b in _L_lock_748 () from /lib/i686/cmov/libpthread.so.0
#3  0xb7702e61 in pthread_mutex_lock () from /lib/i686/cmov/libpthread.so.0
#4  0xb78646ca in LockSemaphoreInfo () from /usr/lib/libMagick.so.10
#5  0xb7864b13 in AcquireSemaphoreInfo () from /usr/lib/libMagick.so.10
#6  0xb782b3f0 in DestroyModuleList () from /usr/lib/libMagick.so.10
#7  0xb78274d7 in DestroyMagickList () from /usr/lib/libMagick.so.10
#8  0xb7827585 in MagickCoreTerminus () from /usr/lib/libMagick.so.10
#9  0xb77fb294 in ?? () from /usr/lib/libMagick.so.10
#10 0xb77fafb2 in MagickFatalError () from /usr/lib/libMagick.so.10
#11 0xb77fb923 in CatchException () from /usr/lib/libMagick.so.10
#12 0xb782a733 in GetModuleInfo () from /usr/lib/libMagick.so.10
#13 0xb7826791 in GetMagickInfo () from /usr/lib/libMagick.so.10
#14 0xb7826a43 in MagickCoreGenesis () from /usr/lib/libMagick.so.10
#15 0xb79c590f in MagickWandGenesis () from /usr/lib/libWand.so.10
#16 0x0828ac02 in imagemagick_load_image (f=0x92629f8, img=0x9245308, 
    contents=0x0, size=0, 
    filename=0x94ac6e0 "/tmp/monnier/docview20848/outsideinX-R1.pdf-cd27e38e9b0bb24741e3d0e767d575f4/page-2.png") at image.c:7526
#17 0x0828bea3 in imagemagick_load (f=0x92629f8, img=0x9245308)
    at image.c:7850
#18 0x08280788 in lookup_image (f=0x92629f8, spec=...) at image.c:1764
#19 0x080a0fb0 in handle_single_display_spec (it=0xbfffc0a0, spec=..., 
    object=..., overlay=..., position=0xbfffc148, display_replaced_before_p=0)
    at xdisp.c:4468
#20 0x0809e723 in handle_display_prop (it=0xbfffc0a0) at xdisp.c:4079
#21 0x0809b96e in handle_stop (it=0xbfffc0a0) at xdisp.c:3154
#22 0x080a628f in reseat (it=0xbfffc0a0, pos=..., force_p=1) at xdisp.c:5598
#23 0x0809ac13 in init_iterator (it=0xbfffc0a0, w=0x934f9d8, 
    charpos=12345678, bytepos=12345678, row=0x93d63b8, 
    base_face_id=DEFAULT_FACE_ID) at xdisp.c:2864
#24 0x0809af43 in start_display (it=0xbfffc0a0, w=0x934f9d8, pos=...)
    at xdisp.c:2880
#25 0x080cf0ad in try_window (window=..., pos=..., flags=1) at xdisp.c:14735
#26 0x080cd141 in redisplay_window (window=..., just_this_one_p=0)
    at xdisp.c:14357
#27 0x080c4df5 in redisplay_window_0 (window=...) at xdisp.c:12576
#28 0x083f1d91 in internal_condition_case_1 (
    bfun=0x80c4dbe <redisplay_window_0>, arg=..., handlers=..., 
    hfun=0x80c4d94 <redisplay_window_error>) at eval.c:1521
#29 0x080c4d76 in redisplay_windows (window=...) at xdisp.c:12556
#30 0x080c2247 in redisplay_internal (preserve_echo_area=1) at xdisp.c:12124
#31 0x080c3d53 in redisplay_preserve_echo_area (from_where=5) at xdisp.c:12388
#32 0x082e8732 in read_char (commandflag=1, nmaps=3, maps=0xbfffe990, 
    prev_event=..., used_mouse_menu=0xbfffead0, end_time=0x0)
    at keyboard.c:2558
#33 0x0830557b in read_key_sequence (keybuf=0xbfffedd0, bufsize=30, 
    prompt=..., dont_downcase_last=0, can_return_switch_frame=1, 
    fix_current_buffer=1) at keyboard.c:9422
#34 0x082e4e24 in command_loop_1 () at keyboard.c:1607
#35 0x083f1c56 in internal_condition_case (bfun=0x82e4471 <command_loop_1>, 
    handlers=..., hfun=0x82e39be <cmd_error>) at eval.c:1476
#36 0x082e3f9b in command_loop_2 (ignore=...) at keyboard.c:1327
#37 0x083f139b in internal_catch (tag=..., func=0x82e3f6b <command_loop_2>, 
    arg=...) at eval.c:1214
#38 0x082e3f40 in command_loop () at keyboard.c:1306
#39 0x082e2fc9 in recursive_edit_1 () at keyboard.c:929
#40 0x082e3580 in Frecursive_edit () at keyboard.c:991
#41 0x082e08b8 in main (argc=5, argv=0xbffff5b4) at emacs.c:1749