Way to reproduce bug:
1. Start sylpheed.
2. Open "configuration/other preferences" menu
3. Open "Message view/External programs" submenu... while opening it crashes.


Configured with: ./configure --prefix=/usr --infodir=/usr/share/info 
--mandir=/usr/share/man --enable-shared --disable-static --disable-rpath 
--disable-nls --disable-ipv6 --disable-trayicon-plugin 
--enable-image-viewer-plugin --enable-gdk-pixbuf --enable-imlib 
--enable-crash-dialog

Sylpheed version 0.9.8claws42  (0.9.8claws26 has same bug)
GTK+ version 1.2.10
Features: gdk-pixbuf
Operating system: Linux 2.4.20-wolk4.0s (i686)
C Library: GNU libc 2.2.5
--
(no debugging symbols found)...[New Thread 1024 (LWP 4740)]
0x4cc3cd99 in wait4 () from /lib/libc.so.6
#0  0x4cc3cd99 in wait4 () from /lib/libc.so.6
#1  0x4ccba30c in __DTOR_END__ () from /lib/libc.so.6
#2  0x4c919ed6 in waitpid () from /lib/libpthread.so.0
#3  0x0809bc3f in crash_main ()
#4  0x4c91770b in pthread_sighandler () from /lib/libpthread.so.0
#5  <signal handler called>
#6  0x4cc08bc1 in strlen () from /lib/libc.so.6
#7  0x4c9a9f31 in gtk_entry_set_text (entry=0x829f858, text=0x0) at gtkentry.c:
447
#8  0x08103d5f in prefs_ext_prog_create_widget ()
#9  0x08152cf5 in pluginwindow_create ()
#10 0x4c9d01f7 in gtk_marshal_NONE__POINTER_INT (object=0xfffffe00, 
    func=0x8152bc0 <pluginwindow_create+2320>, func_data=0xfffffe00, args=0x0) 
at gtkmarshal.c:325
#11 0x4c9f6625 in gtk_handlers_run (handlers=0x82828b8, signal=0xbfad0d50, 
object=0x8285148, 
    params=0xbfad0dc0, after=0) at gtksignal.c:1912
#12 0x4c9f5c40 in gtk_signal_real_emit (object=0x8285148, signal_id=82, 
params=0xbfad0dc0)
    at gtksignal.c:1477
#13 0x4c9f4359 in gtk_signal_emit (object=0xfffffe00, signal_id=82) at 
gtksignal.c:552
#14 0x4c99dc99 in real_select_row (clist=0x52, row=-512, column=-512, 
event=0x828c670)
    at gtkctree.c:3297
#15 0x4c9d01ae in gtk_marshal_NONE__INT_INT_POINTER (object=0xfffffe00, 
    func=0x4c99dc20 <real_select_row>, func_data=0xfffffe00, args=0x0) at 
gtkmarshal.c:300
#16 0x4c9f5b55 in gtk_signal_real_emit (object=0x52, signal_id=3215789504, 
params=0xbfad1160)
    at gtksignal.c:1440
#17 0x4c9f4359 in gtk_signal_emit (object=0xfffffe00, signal_id=61) at 
gtksignal.c:552
#18 0x4c98a54d in gtk_clist_button_press (widget=0x3d, event=0x828c670) at 
gtkclist.c:5066
#19 0x4c999082 in gtk_ctree_button_press (widget=0x8285148, event=0x828c670) at 
gtkctree.c:821
#20 0x4c9cfe16 in gtk_marshal_BOOL__POINTER (object=0xfffffe00, 
    func=0x4c999030 <gtk_ctree_button_press>, func_data=0xfffffe00, args=0x0) at 
gtkmarshal.c:28
#21 0x4c9f5c85 in gtk_signal_real_emit (object=0x8285148, signal_id=4, 
params=0xbfad1570)
    at gtksignal.c:1492
#22 0x4c9f4359 in gtk_signal_emit (object=0xfffffe00, signal_id=20) at 
gtksignal.c:552
#23 0x4ca22f0a in gtk_widget_event (widget=0x14, event=0xbfad1570) at gtkwidget.
c:2864
#24 0x4c9cfc97 in gtk_propagate_event (widget=0x8285148, event=0x828c670) at 
gtkmain.c:1392
#25 0x4c9cecc6 in gtk_main_do_event (event=0x828c670) at gtkmain.c:818
#26 0x4ca563d3 in gdk_event_dispatch (source_data=0x0, current_time=0xbfad1930, 
user_data=0x0)
    at gdkevents.c:2139
#27 0x4ca7f70e in g_main_dispatch (dispatch_time=0xbfad1930) at gmain.c:656
#28 0x4ca7fbc0 in g_main_iterate (block=1, dispatch=1) at gmain.c:877
#29 0x4ca7fdea in g_main_run (loop=0x82898a8) at gmain.c:935
#30 0x4c9ce7cf in gtk_main () at gtkmain.c:524
#31 0x080d0efb in main ()
#32 0x4cb9f4d2 in __libc_start_main () from /lib/libc.so.6