The race condition happens between seek() and state_changed_cb()

Under some conditions, the state_changed_cb() can end up removing the seek_source while it is being created and setup by seek(). This will lead to a null pointer de-reference in seek().

Adding a sleep (of, say, 250ms) after 
   self->seek_source = g_timeout_source_new (delay);
will greatly increase the probability of triggering the race condition.

Taking the player lock while creating and setting up the seek_source seems to address the issue.

I'll submit a patch later.