avformat/rmdec: when reading audio blocks, dont leave holes when reading fails

The fate test is changed because the reference file depends on the use of
non cleared data at the very
end. Alternatively we could upload a new reference file, though that would
then have to be changed every time the handling of a truncated frame changes
or theres a change to error concealment, each time adding a new file ...

Fixes use of uninitialized memory
Fixed: msan_uninit-mem_7f3c02b81363_2787_RLG2_19.rm
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
