assert client in mongoc_bulk_operation_set_client

This function should not be used to set the client to NULL and an assertion
also guards against a potential segfault when incrementing operation_id.

Author: jmikola

src/mongoc/mongoc-bulk-operation.c

@@ -839,6 +839,7 @@ void
 mongoc_bulk_operation_set_client (mongoc_bulk_operation_t *bulk, void *client)
 {
    BSON_ASSERT (bulk);
+   BSON_ASSERT (client);
 
    bulk->client = (mongoc_client_t *) client;