Jes/uac registrant null pointer dereference

[jes]

Summary
I was seeing segfaults when trying to make opensips register against a server with a much-too-short interval. I traced the problem back to a uac_registrant null pointer dereference of msg->min_expires.

Details
This commit makes uac_registrant only try to dereference msg->min_expires if it is non-NULL.

Solution
I wrapped the dereference of msg->min_expires in if (msg->min_expires).

Compatibility
It is possible that we would instead want parse_min_expires() to return nonzero if msg->min_expires is NULL. I couldn't quite work out what logic was intended. I think the code in this PR will fix the segfault, but it may not correct the intended behaviour.

Closing issues

[bogdan-iancu]

Hi @jes , imho the proper fix would be to have parse_min_expires() returning false if there is no hdr or if it failed to parse.

[bogdan-iancu]

@jes , see me alternative commit - if works for you, I will backport and close here.

[jes]

@bogdan-iancu looks good to me!

It solves the segfault, and the logic looks fine for the uac_registrant case. I didn't check if anything else is relying on the old behaviour with parse_min_expires() returning 0 with msg->min_expires == NULL, but it seems unlikely and you're more likely to know than me anyway.

[bogdan-iancu]

Thanks @jes for the confirmation, the backports are in place, closing here