fix: Prevent null dereference in AITNGuardOuterState update

[sentry]

This PR addresses a fatal crash (EXCEPTION_ACCESS_VIOLATION_READ) occurring in AITNGuardOuterState::update().

The root cause was identified as a null pointer dereference of m_attackState. The onEnter() method for AITNGuardOuterState can return early with STATE_SUCCESS if the guard mode is GUARDMODE_GUARD_WITHOUT_PURSUIT or if the nemesis object is null. In these scenarios, m_attackState is never initialized, remaining nullptr.

Subsequently, when AITNGuardOuterState::update() is called, it attempts to access m_attackState->getMachineGoalObject() without first checking if m_attackState is valid, leading to a crash.

The fix introduces a null-check at the beginning of AITNGuardOuterState::update(). If m_attackState is nullptr, the method now safely returns STATE_SUCCESS, aligning its behavior with the early-exit conditions of onEnter() and preventing the dereference of an uninitialized pointer.

Fixes CLIENT-19W