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/971088Signed-off-by: Bernhard Übelacker <bernhardu@mailbox.org>
(cherry picked from commit c15dd0ba)

os/backtrace.c

@@ -45,6 +45,7 @@ xorg_backtrace(void)
 {
     unw_cursor_t cursor;
     unw_context_t context;
+    unw_word_t ip;
     unw_word_t off;
     unw_proc_info_t pip;
     int ret, i = 0;
@@ -88,7 +89,9 @@ xorg_backtrace(void)
             procname[1] = 0;
         }
 
-        if (dladdr((void *)(uintptr_t)(pip.start_ip + off), &dlinfo) && dlinfo.dli_fname &&
+        if (unw_get_reg (&cursor, UNW_REG_IP, &ip) < 0)
+          ip = pip.start_ip + off;
+        if (dladdr((void *)(uintptr_t)(ip), &dlinfo) && dlinfo.dli_fname &&
                 *dlinfo.dli_fname)
             filename = dlinfo.dli_fname;
         else
@@ -96,7 +99,7 @@ xorg_backtrace(void)
 
         ErrorFSigSafe("%u: %s (%s%s+0x%x) [%p]\n", i++, filename, procname,
             ret == -UNW_ENOMEM ? "..." : "", (int)off,
-            (void *)(uintptr_t)(pip.start_ip + off));
+            (void *)(uintptr_t)(ip));
 
         ret = unw_step(&cursor);
         if (ret < 0)