os: Fix instruction pointer written in xorg_backtrace

The address retrieved in "pip.start_ip" is not necessarily the same
address as unw_get_proc_name finds as nearest symbol and returns in "off".
Therefore using "pip.start_ip + off" is not reliable, at least
visible in the binaries from the Debian repository.

Bug-Debian: https://bugs.debian.org/971088

Signed-off-by: Bernhard Übelacker <bernhardu@mailbox.org>
(cherry picked from commit c15dd0ba)
3 jobs for server-1.20-branch in 6 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #5327786
debian-testing

00:00:12

 
  Build And Test
passed #5327787
autotools-build-and-test

00:06:23

passed #5327788
meson-build-and-test

00:02:12