I copied __ptr_oob* checks from util-vdso.c.
That will allow to check not only structure begin pointers,
but structure end also.
Before this patch code checked if strings section's header pointer is OOB,
but it did it after dereferencing this pointer, which is meaningless.

Now it checks:
- OOB of sections table,
- strings section's header should be inside sections table,
- check strings section with it's length for OOB.

In the very next patches I will add tests for this functions and
other compel-related changes.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>