ituh263: reject b-frame with pp_time = 0

Avoid a division by 0 in ff_mpeg4_set_one_direct_mv.

Sample-Id: 00000168-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
(cherry picked from commit 9514440337875e0c63b409abcd616b68c518283f)
(cherry picked from commit 5df52b0131d3d4d804ad6e221bc9a2cd8b201ef2)
configure: enable PIC on s390(x)

The s390 architecture requires shared libraries to be built in PIC mode.
Otherwise applications will get wrong relocations at run-time, leading
to confusing segmentation faults.

CC: libav-stable@libav.org
(cherry picked from commit 5ddc9f5052316608799b932c604f9e7561f8ce24)
(cherry picked from commit 7509c2c4ea2180733cc60ab1a0e0fe4ce2f02a69)