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>