shorten: Use separate pointers for the allocated memory for decoded samples.

Fixes invalid free() if any of the buffers are not allocated due to either
not decoding a header or an error prior to allocating all buffers.

Fixes CVE-2012-0858
CC: libav-stable@libav.org

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>
