base-modem: don't assume ports tables always exist

The 'link_ports' and 'ports' tables are created during object init(), but they are fully removed and cleared during dispose(). Given that the MMDevice executes an explicit g_object_run_dispose(), there may be cases where a long-running operation that isn't cancelled ends up being completed by the time the object disposal has already run at least once.

That would end up crashing the process if we attempt to e.g. iterate over one of the ports hash tables:

  0x00007c3594eb3b93 (libglib-2.0.so.0 - ghash.c: 1180)   g_hash_table_iter_next
  0x00005b60d86f2563 (ModemManager - mm-base-modem.c: 1133)       mm_base_modem_get_port_infos
  0x00005b60d870228e (ModemManager - mm-iface-modem.c: 4013)      fcc_unlock
  0x00007c35950126a8 (libgio-2.0.so.0 - gtask.c: 1230)    g_task_return_now
  0x00007c35950116fa (libgio-2.0.so.0 - gtask.c: 1300)    g_task_return
  0x00007c3595011d12 (libgio-2.0.so.0 - gtask.c: 1930)    g_task_return_new_error
  0x00005b60d87518f2 (ModemManager - mm-broadband-modem-mbim.c: 1970)     radio_state_set_up_ready
  0x00007c35950126a8 (libgio-2.0.so.0 - gtask.c: 1230)    g_task_return_now
  0x00007c35950116fa (libgio-2.0.so.0 - gtask.c: 1300)    g_task_return
  0x00007c35950c486c (libmbim-glib.so.4 - mbim-device.c: 253)     transaction_task_complete_and_free
  0x00007c35950c714b (libmbim-glib.so.4 - mbim-device.c: 335)     transaction_timed_out
  0x00007c3594ec1232 (libglib-2.0.so.0 - gmain.c: 4971)   g_timeout_dispatch
  0x00007c3594ec43fc (libglib-2.0.so.0 - gmain.c: 3417)   g_main_context_dispatch
  0x00007c3594ec4704 (libglib-2.0.so.0 - gmain.c: 4211)   g_main_context_iterate
  0x00007c3594ec4978 (libglib-2.0.so.0 - gmain.c: 4411)   g_main_loop_run
  0x00005b60d86d7c56 (ModemManager - main.c: 217) main
  0x00007c3594c526c5 (libc.so.6 + 0x000286c5)     __libc_init_first
  0x00007c3594c52781 (libc.so.6 + 0x00028781)     __libc_start_main
  0x00005b60d86d7970 (ModemManager + 0x00061970)  _start

Fixes #677 (closed)

Change-Id: I6695c284f86a196e60de7f714bc1671332d08848