libkmod: Fix overflow in kmod_module_hex_to_str

libkmod: Fix overflow in kmod_module_hex_to_str

If an overly long signature is found in a module file, it is possible to
trigger an out of boundary write in kmod_module_hex_to_str due to
integer and subsequent heap buffer overflow.

This approach replaces malloc + sprintf with a simple hex-lookup and a
strbuf approach, being slightly faster in real life scenarios while
adding around 100 bytes to library size. A much faster approach could be
done without strbuf and using our overflow check functions, but
readability should win here.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Link: https://github.com/kmod-project/kmod/pull/236
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>