CDRIVER-5727 zero parts->assembled.payload (#1731)

To silence Coverity warning of uninitialized variable. See issue with CID 118911.

This is applied from the commit e45fd98 on r1.27. The commit on r1.27 appears to have been made without an associated commit on master.

Author: kevinAlbs

src/libmongoc/src/mongoc/mongoc-cmd.c

@@ -56,6 +56,7 @@ mongoc_cmd_parts_init (mongoc_cmd_parts_t *parts,
    parts->assembled.query_flags = MONGOC_QUERY_NONE;
    parts->assembled.op_msg_is_exhaust = false;
    parts->assembled.payloads_count = 0;
+   memset (parts->assembled.payloads, 0, sizeof parts->assembled.payloads);
    parts->assembled.session = NULL;
    parts->assembled.is_acknowledged = true;
    parts->assembled.is_txn_finish = false;