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