coverity fixes for bntest.c

Reviewed-by: NPaul Yang <yang.yang@baishancloud.com>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8508)

(cherry picked from commit e2f5081116d8b3dadd602fcf611bc6584ab395f7)