my/hex_decode: don't allow use-after-free in error path
my/hex_decode: don't allow use-after-free in error path

in the error case, where the input passed to hex_decode() is invalid,
make sure we don't allow a stray pointer to escape via the 'raw'
parameter-return value. this prevents use-after-free() in callers.

farsightsec/fstrm,snarkmaster/mtbl,snarkmaster/mtbl,farsightsec/mtbl,farsightsec/fstrm,rep/mtbl,snarkmaster/mtbl,rep/mtbl,farsightsec/mtbl,rep/mtbl