base-modem: fix segfault when disposing modem

If we have a modem reference around during an ongoing operation but
the modem has already been disposed after getting removed from the
system, we were trying to iterate a NULL hash table, which led to a
crash.

https://lists.freedesktop.org/archives/modemmanager-devel/2018-November/006915.html

Reported by Sebastien Fabre <sebastien.fabre@sigfox.com>

src/mm-base-modem.c

@@ -959,6 +959,9 @@ mm_base_modem_find_ports (MMBaseModem *self,
     gpointer value;
     gpointer key;
 
+    if (!self->priv->ports)
+        return NULL;
+
     /* We'll iterate the ht of ports, looking for any port which is matches
      * the compare function */
     g_hash_table_iter_init (&iter, self->priv->ports);