Fixed assertion when sip error code is not set (thanks Peter Koletzki for the report)

[ming]

PJSIP is throwing an assertion at sip_util.c:497 when during executing pjsua_start(), an incoming call is received.

This is caused by:

1. pjsua_call.c / pjsua_call_on_incoming():

sip_err_code is uninitialized

2. pjsua_media.c / pjsua_media_channel_init()

when pjsua_get_status() != RUNNING, the function will return without setting the sip_err_code.

This creates a response with random sip error code and an assertion is thrown.