Don't raise assert when receiving an incoming call without a pjsua account

[riza]

When there is no pjsua account available, and incoming call is received, assertion is raised:

pjsua_acc_find_for_incoming: assertion "pjsua_var.acc_cnt!=0" failed

In this case, assertion should be avoided since it is an uncontrollable event.

Thanks to Stanley Knapczyk for the report.

[riza]

In 5805:

Re #2119 (closed): Don't raise assert when receiving an incoming call with no
pjsua account available.