Security fixes in SDP code

[lminiero]

We've been notified about a series of security issues related to SDP (see relevant CVE IDs below, vulnerabilities discovered by Mikhail Evdokimov at Digital Security dsec.ru). Specifically, each commit in this PR solves a different issue that could happen:

1. a NULL pointer dereference when parsing some SDP attributes (see CVE-2020-13898);
2. a stack memory leak (and a "regular" one) when pre-parsing an SDP (see CVE-2020-13899);
3. a different NULL pointer dereference when pre-parsing an SDP (see CVE-2020-13900);
4. a buffer overflow when merging SDPs (see CVE-2020-13901).

I plan to merge this almost immediately, as I can't replicate the issues anymore.