nonblock: call with (void) to show we ignore the return code

Coverity pointed out several of these.
Curl_sec_read_msg: spell out that we ignore return code

Coverity CID 1241947. Since if sscanf() fails, the previously set value
remains set.
choose_mech: fix return code

Coverity CID 1241950. The pointer is never NULL but it might point to
NULL.