os: avoid uninitialized offsets in backtrace

When xorg_backtrace calls unw_get_proc_name and an error occurs, offset
might not be set for the current frame.

Initialize offset for each frame so that the offset from another frame
cannot be used inadvertently.

Signed-off-by: Jeff Smith <whydoubt@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

os/backtrace.c

@@ -78,6 +78,7 @@ xorg_backtrace(void)
             break;
         }
 
+        off = 0;
         ret = unw_get_proc_name(&cursor, procname, 256, &off);
         if (ret && ret != -UNW_ENOMEM) {
             if (ret != -UNW_EUNSPEC)