FIX strncpy warning in apps/cmp.c.

bugfix: #12872

strncpy here has compiling warning of -Wstringop-truncation, change
into BIO_snprintf as before.

Change-Id: I362872c4ad328cadd4c7a5a5da3165655fa26c0d

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/12889)
