alac: Limit max_samples_per_frame

Otherwise buffer size calculations in allocate_buffers could
overflow later, making the code think a large enough buffer
actually was allocated.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
Merge commit 'f7c5883126f9440547933eefcf000aa78af4821c'

* commit 'f7c5883126f9440547933eefcf000aa78af4821c':
  alac: Limit max_samples_per_frame

See: 3920d1387834e2bc334aff9f518f4beb24e470bd
Merged-by: Michael Niedermayer <michaelni@gmx.at>