Fix memory out-of-bounds access when ex_bits is 0 in HNSW search

[Light-V]

Description

This PR fixes a memory out-of-bounds access bug in the HNSW search functionality.

Problem

When ex_bits_ is 0, the get_full_est function accesses non-existent ex_data, causing out-of-bounds memory access.

Solution

Added a check before calling get_full_est in searchBaseLayerST_AdaptiveRerankOpt. When ex_bits_ is 0, we use get_bin_est instead, which is the correct behavior since there is no additional refinement data available.

Changes

• Modified searchBaseLayerST_AdaptiveRerankOpt to check ex_bits_ before calling get_full_est

Testing

The fix has been tested locally and resolves the issue without introducing performance overhead in the common case (ex_bits > 0).