BUILD: pool: Fix GCC error about potential null pointer dereference

BUILD: pool: Fix GCC error about potential null pointer dereference

In pool_gc(), GCC 13.2.1 reports an error about a potential null potential
dereference:

src/pool.c: In function ‘pool_gc’:
src/pool.c:807:64: error: potential null pointer dereference [-Werror=null-dereference]
  807 |                         entry->buckets[bucket].free_list = temp->next;
      |                                                            ~~~~^~~~~~

There is no issue here because "bucket" variable cannot be greater than
CONFIG_HAP_POOL_BUCKETS. But to make GCC happy, we now break the loop if it
is greater or equal to CONFIG_HAP_POOL_BUCKETS.