Fix two issues found in flash_spec 'etsi' tests

- R0. If more than one account is registered, and there has another incoming call(ringing) to other accounts,
  it will be rejected as well when the ongoing one doing R0. (A, B registered on DUT; A<=>C(ongoing call);
  D=>B(ringing); E=>A(ringing,cw); A press R0 then both D and E be rejected)
- 3-way conference back to 2-way call, then performing R1 will lead to a crash