libavutil/channel_layout: Check strtol*() for failure

Fixes assertion failure
Fixes: 4f5814bb15d2dda6fc18ef9791b13816/signal_sigabrt_7ffff6ae7cc9_65_7209d160d168b76f311be6cd64a548eb.wv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>