file_util.c: refactor and fix checksum_hex2bin()

file_util.c: refactor and fix checksum_hex2bin()

 - Simplify function body by reducing the amount of intermediate vars
 - Derive length checks from the size of the buf
 - Replace complex for(;;) by a simple while() advancing the src string
 - Handle edge case of a zero length input not returning NULL

Fixes: c09fe20 ("libopkg: fix skipping of leading whitespace when parsing checksums")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>