Fix memory leaks on failed ff_h264_decode_init()

During failure conditions ff_h264_decode_init() leaks memory
allocated for nal units.  Found via valgrind.

Valgrind traces: http://pastebin.com/GqTqxs8T

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
