[OpenSIPS-Devel] [OpenSIPS/opensips] c990e6: Fix crash in parse_sdp when a= is empty

[Răzvan Crainea]

  Branch: refs/heads/3.1
  Home:   https://github.com/OpenSIPS/opensips
  Commit: c990e6ab1f6b301b172d7886ca290beb6db7a7b9
      https://github.com/OpenSIPS/opensips/commit/c990e6ab1f6b301b172d7886ca290beb6db7a7b9
  Author: Razvan Crainea <razvan at opensips.org>
  Date:   2021-10-08 (Fri, 08 Oct 2021)

  Changed paths:
    M parser/sdp/sdp_helpr_funcs.c

  Log Message:
  -----------
  Fix crash in parse_sdp when a= is empty

When a bogus SDP was provided, with an empty `a=` line, there was no
check for the length to be compared, resulting in a bad memory access,
hence a crash.

Issue discovered during OpenSIPS Security Audit 2021,
    by Alfred Farrugia & Sandro Gauci (Enable Security)

(cherry picked from commit 2617c97207b1fe2afead9f887f7a5df4da3b7d55)