Fix compiler warning (#1254)

GCC 13 warns that `pool` may be used uninitialized. This is a false
positive because pool will only be used in the case that it is
initialized, but by initializing it to NULL we can avoid this warning.

Author: Kyle Kloberdanz

src/libmongoc/tests/test-mongoc-client.c

@@ -479,7 +479,7 @@ test_mongoc_client_speculative_auth_failure (bool pooled)
    char *uri_str_auth;
    mongoc_collection_t *collection;
    mongoc_client_t *auth_client;
-   mongoc_client_pool_t *pool;
+   mongoc_client_pool_t *pool = NULL;
    mongoc_uri_t *uri;
    mongoc_cursor_t *cursor;
    const bson_t *doc;