Make comm_timer_disable() safe to call repeatedly

[rtrappman-dev]

Summary

Make comm_timer_disable() idempotent by deleting the underlying event only when the timer is currently enabled.

The existing implementation unconditionally called ub_timer_del(), even when the timer had already been disabled. Repeated disable/delete sequences could therefore attempt to remove an event more than once, potentially causing event corruption or use-after-free behavior.

Changes

• Check timer->ev_timer->enabled before calling ub_timer_del().
• Clear the enabled state only when an active timer is actually disabled.
• Preserve the existing NULL-timer handling.

Security / Reliability Impact

This hardens the asynchronous event lifecycle against double deletion of timer events.

comm_timer_disable() is used across timeout, retry, cleanup, and error-handling paths, where repeated disable operations can legitimately occur. Making the operation idempotent prevents invalid event deletion and reduces the risk of event-state corruption and use-after-free conditions.

Testing

The change was verified against the fix/comm_timer_disable() branch and preserves the existing timer lifecycle semantics:

• NULL timers remain safely ignored.
• Active timers are disabled and have their event removed.
• Already-disabled timers are left untouched.