avcodec/alac: only set *got_frame_ptr when all channels have been decoded

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f8b64436530_7895_quicktime_newcodec_applelosslessaudiocodec.m4a
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
