Failed re-invite/update caused by a bug in provisional media clean up

[ming]

Consider this scenario:

1. send re-Invite with video
2. get 491 resp
3. send ACK
4. send re-Invite with video

Currently, the second re-Invite will fail due to the bug in provisional media cleanup.

Clean up provisional media upon:

• failed (non-2xx) responses, or
• 2xx responses but no/invalid answer

Additionally, the ticket will fix media_prov_clean_up() to clear
call->med_prov_cnt, so that pjsua_call_media_is_changing() can behave
correctly.

[ming]

In 4948:

Fixed #1797: Clean up provisional media after re-invite/update

[riza]

In 4956:

Fixed #1797: Clean up provisional media is executed when there is an ongoing SDP negotiation.
Scenario:

1. re-invite got 491, state tsx changed to completed
2. cleanup prov med executed
3. re-invite again with video
4. timer tsx to shift tsx no 1, from completed -> terminated trigger provisional cleanup
5. provisional video transport in no 3 got cleanup

[ming]

In 4976:

Re #1797: Fixes assert if 422 response is received (thanks to Brian Walker for the report)

If 422 is received, INVITE will be automatically retried, but the
provisional media has been cleaned up.