fix: release EtherCAT master in EcMaster destructor

[apajon]

Problem

The current jazzy branch correctly defers EcMaster initialization, so multi-master setups using master_id are already supported.

However, EcMaster does not explicitly release the IgH master in its destructor. This can leave the master reserved after shutdown and may interfere with clean restart or re-acquisition.

Solution

Add an explicit ecrt_release_master(master_) call in ~EcMaster():

if (master_ != NULL) {
  ecrt_release_master(master_);
  master_ = NULL;
}

This ensures proper cleanup and symmetry with ecrt_request_master().

Impact

• improves shutdown behavior
• avoids master remaining reserved after process exit
• no impact on runtime behavior

Why this PR

This PR intentionally focuses on a minimal and isolated fix for proper resource cleanup in the current jazzy branch.

Testing

Validated in a real setup (ROS2 Jazzy / Ubuntu 24.04) with repeated startup / shutdown cycles on a multi-bus EtherCAT configuration.