Avoid agressive may be used uninitialized warning

tests/test-mongoc-exhaust.c:242:7: warning: ‘pool’ may be used
uninitialized in this function [-Wmaybe-uninitialized]

Author: bjori

tests/test-mongoc-exhaust.c

@@ -43,7 +43,7 @@ test_exhaust_cursor (bool pooled)
    mongoc_stream_t *stream;
    mongoc_write_concern_t *wr;
    mongoc_client_t *client;
-   mongoc_client_pool_t *pool;
+   mongoc_client_pool_t *pool = NULL;
    mongoc_collection_t *collection;
    mongoc_cursor_t *cursor;
    mongoc_cursor_t *cursor2;