Replace snprintf with strncpy at some places to avoid truncation

Replace snprintf with strncpy at some places to avoid truncation

In gcc7 there are some building errors like:
directive output may be truncated writing up to 31 bytes into a region of size 24
snprintf(str, MPB_SIG_LEN, %s, mpb->sig);

It just need to copy one string to target. So use strncpy to replace it.

For this line code: snprintf(str, MPB_SIG_LEN, %s, mpb->sig);
Because mpb->sig has the content of version after magic, so
it's better to use strncpy to replace snprintf too.

Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@gmail.com>