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printing: Fix crash in avahi_service_resolver_cb

WGH requested to merge WGH-/gtk:fix-avahi_service_resolver_cb-crash into gtk-3-24

printer_name_compressed_strv is NULL-terminated array of gchar*, which means N+1 memory should be allocated.

Otherwise, if the printer name has no empty components (which is usually the case), printer_name_compressed_strv[N], which should contain the NULL sentinel, will actually lie just outside of allocated memory, which is UB.

In my case, it led to crashes inside g_strjoinv when Print... dialog is opened in evince.

#0  0x00007fad2ce1bad7 in __strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:96
#1  0x00007fad2d04d88d in g_strjoinv (separator=separator@entry=0x7fad0c9bc508 "-", str_array=str_array@entry=0x556b017f0200) at ../glib-2.60.7/glib/gstrfuncs.c:2585
#2  0x00007fad0c9b8a89 in avahi_service_resolver_cb (source_object=<optimized out>, res=<optimized out>, user_data=0x7fad08020ee0) at /var/tmp/portage/x11-libs/gtk+-3.24.13/work/gtk+-3.24.13/modules/printbackends/cups/gtkprintbackendcups.c:3223
#3  0x00007fad2d1f8ed3 in g_task_return_now (task=0x556b017a8b00 [GTask]) at ../glib-2.60.7/gio/gtask.c:1209
#4  0x00007fad2d1f987d in g_task_return (task=0x556b017a8b00 [GTask], type=<optimized out>) at ../glib-2.60.7/gio/gtask.c:1278
#5  0x00007fad2d1f9dec in g_task_return (type=G_TASK_RETURN_SUCCESS, task=<optimized out>) at ../glib-2.60.7/gio/gtask.c:1678
#6  0x00007fad2d1f9dec in g_task_return_pointer (task=<optimized out>, result=<optimized out>, result_destroy=<optimized out>) at ../glib-2.60.7/gio/gtask.c:1683
#7  0x00007fad2d24b6af in g_dbus_connection_call_done (source=<optimized out>, result=0x556b017a8bc0, user_data=0x556b017a8b00) at ../glib-2.60.7/gio/gdbusconnection.c:5747
#8  0x00007fad2d1f8ed3 in g_task_return_now (task=0x556b017a8bc0 [GTask]) at ../glib-2.60.7/gio/gtask.c:1209
#9  0x00007fad2d1f8f09 in complete_in_idle_cb (task=0x556b017a8bc0) at ../glib-2.60.7/gio/gtask.c:1223
#10 0x00007fad2d02d2c0 in g_main_dispatch (context=0x556b00eee090) at ../glib-2.60.7/glib/gmain.c:3189
#11 0x00007fad2d02d2c0 in g_main_context_dispatch (context=context@entry=0x556b00eee090) at ../glib-2.60.7/glib/gmain.c:3854
#12 0x00007fad2d02d658 in g_main_context_iterate (context=context@entry=0x556b00eee090, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib-2.60.7/glib/gmain.c:3927
#13 0x00007fad2d02d6df in g_main_context_iteration (context=context@entry=0x556b00eee090, may_block=may_block@entry=1) at ../glib-2.60.7/glib/gmain.c:3988
#14 0x00007fad2d22248d in g_application_run (application=0x556b0116f130 [EvApplication], argc=<optimized out>, argv=<optimized out>) at ../glib-2.60.7/gio/gapplication.c:2519
#15 0x0000556b002e55a1 in  ()
#16 0x00007fad2ccd6f1b in __libc_start_main (main=0x556b002e50d0, argc=2, argv=0x7ffe1057fa88, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffe1057fa78) at ../csu/libc-start.c:308
#17 0x0000556b002e567a in  ()

(gdb) p printer_name_compressed_strv[0]
$4 = (gchar *) 0x556d4a4be430 "Brother"
(gdb) p printer_name_compressed_strv[1]
$5 = (gchar *) 0x7f9dbc011090 "MFC"
(gdb) p printer_name_compressed_strv[2]
$6 = (gchar *) 0x556d4a51ba50 "7860DW"
(gdb) p printer_name_compressed_strv[3]
$7 = (gchar *) 0x401 <error: Cannot access memory at address 0x401>

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