cmp_mock_srv.c: Fix polling mode such that it can be done multiple times
cmp_mock_srv.c: Fix polling mode such that it can be done multiple times

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15848)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl