Assertion if ending 100rel session when invite on early state

[ming]

As reported by Brian Walker. Stack trace:
pjsip_100rel_end_session (in MobileClient?) + 91
inv_set_state (in MobileClient?) + 211
inv_on_state_early (in MobileClient?) + 685
mod_inv_on_tsx_state (in MobileClient?) + 69
pjsip_dlg_on_tsx_state (in MobileClient?) + 109
tsx_timer_callback (in MobileClient?) + 227
pj_timer_heap_poll (in MobileClient?) + 159
pjsip_endpt_handle_events2 (in MobileClient?) + 47
worker_thread (in MobileClient?) + 65
thread_main (in MobileClient?) + 67

Similar report can be found in the mailing list (March 2014):
​http://lists.pjsip.org/pipermail/pjsip_lists.pjsip.org/2014-March/017290.html