avutil/avstring: add av_strnlen()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/wavpack: clear remainder of data in case of error in wv_unpack_mono/stereo()

Fixes use of uninitialized data
Fixes: msan_uninit-mem_7fd85b654950_4005_because.wv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>