avcodec/mpegaudiodec_template/mp3on4: check that all channels have been decoded before returnig a frame

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f80bf9a259e_5774_id5_1.mp4
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>