broadband-modem: ensure mm_broadband_modem_create_device_identifier() sets an...

broadband-modem: ensure mm_broadband_modem_create_device_identifier() sets an error on all bad returns

modem_load_device_identifier_finish() expect it to do so and trips an
assertion failure in g_propagate_error():

  ModemManager[256038]: <warn>  [1633083601.491190] [modem0] couldn't load equipment identifier: Unknown error
  ModemManager[256038]: <debug> [1633083601.491204] [modem0] loading device identifier...
  ModemManager[256038]: <debug> [1633083601.491215] [modem0/modemu/at] device open count is 3 (open)
  ModemManager[256038]: <debug> [1633083601.491231] [modem0/modemu/at] device open count is 2 (close)
  ModemManager[256038]: <debug> [1633083601.491254] [modem0/modemu/at] --> 'ATI<CR>'
  ModemManager[256038]: <debug> [1633083601.494616] [modem0/modemu/at] <-- '<CR><LF>ERROR<CR><LF>'
  ModemManager[256038]: <debug> [1633083601.494643] [modem0/modemu/at] operation failure: 100 (Unknown error)

  (ModemManager:256038): GLib-CRITICAL **: 06:20:01.494: g_propagate_error: assertion 'src != NULL' failed

  Thread 1 "ModemManager" received signal SIGTRAP, Trace/breakpoint trap.
  g_logv (log_domain=0x7ffff77d1071 "GLib", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=<optimized out>) at ../glib/gmessages.c:1413
  1413		  g_private_set (&g_log_depth, GUINT_TO_POINTER (depth));
  Missing separate debuginfos, use: dnf debuginfo-install libmbim-1.26.0-2.el9.x86_64 libqmi-1.30.2-1.el9.x86_64
  (gdb) bt
  #0  g_logv (log_domain=0x7ffff77d1071 "GLib", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=<optimized out>) at ../glib/gmessages.c:1413
  #1  0x00007ffff77813d3 in g_log (log_domain=<optimized out>, log_level=<optimized out>, format=<optimized out>) at ../glib/gmessages.c:1451
  #2  0x000055555561122d in modem_load_device_identifier_finish (self=0x5555557e62d0, res=<optimized out>, error=0x7fffffffd320)
      at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/mm-broadband-modem.c:1168
  #3  0x00005555555ea69a in load_device_identifier_ready (self=0x5555557e62d0, res=0x5555557ea6b0, task=task@entry=0x7fffe8011e20 [GTask])
      at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/mm-iface-modem.c:4727
  #4  0x00007ffff7951b62 in g_simple_async_result_complete (simple=0x5555557ea6b0 [GSimpleAsyncResult]) at ../gio/gsimpleasyncresult.c:802
  #5  0x00005555555d1b0a in at_sequence_parse_response (port=<optimized out>, res=<optimized out>, ctx=ctx@entry=0x5555557ed7f0)
      at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/mm-base-modem-at.c:250
  #6  0x00007ffff7951b62 in g_simple_async_result_complete (simple=0x5555557e5440 [GSimpleAsyncResult]) at ../gio/gsimpleasyncresult.c:802
  #7  0x00005555556805dc in serial_command_ready (port=<optimized out>, res=<optimized out>, simple=simple@entry=0x5555557e5440 [GSimpleAsyncResult])
      at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/mm-port-serial-at.c:378
  #8  0x00007ffff7951b62 in g_simple_async_result_complete (simple=0x5555557d6730 [GSimpleAsyncResult]) at ../gio/gsimpleasyncresult.c:802
  #9  0x000055555567fb35 in command_context_complete_and_free (ctx=ctx@entry=0x7fffe800c200, idle=idle@entry=0) at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/mm-port-serial.c:141
  #10 0x0000555555682a96 in port_serial_got_response (self=0x5555557dd4e0 [MMPortSerialAt], parsed_response=0x0, error=0x5555557d5040)
      at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/mm-port-serial.c:755
  #11 0x00005555556870d2 in parse_response_buffer (self=<optimized out>) at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/mm-port-serial.c:934
  #12 common_input_available (self=0x5555557dd4e0 [MMPortSerialAt], condition=(G_IO_NVAL | unknown: 0xf762a5c0)) at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/mm-port-serial.c:1035
  #13 0x00007ffff7778f9f in g_main_dispatch (context=0x5555557527c0) at ../glib/gmain.c:3337
  #14 g_main_context_dispatch (context=0x5555557527c0) at ../glib/gmain.c:4055
  #15 0x00007ffff77cd608 in g_main_context_iterate.constprop.0 (context=0x5555557527c0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/gmain.c:4131
  #16 0x00007ffff7778563 in g_main_loop_run (loop=0x55555576da60) at ../glib/gmain.c:4329
  #17 0x00005555555b7b60 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/ModemManager-1.18.2-1.el9.x86_64/src/main.c:216

(cherry picked from commit e3d108de)