AP_HAL_SITL: fix Semaphore ownership race in give()

give() cleared the owner field after releasing the mutex.  In the window between the unlock and the owner=-1 assignment another thread could take the lock and set itself as owner; we would then clobber that ownership, causing a spurious "Wrong owner" panic when the victim thread next called check_owner().

owner is now cleared while the lock is still held, so it is only ever mutated under lock protection.  This is exercised when two threads share a bus semaphore, e.g. a driver with its own thread and a periodic-callback driver on the same simulated SPI bus.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

libraries/AP_HAL_SITL/Semaphores.cpp

@@ -22,12 +22,12 @@ Semaphore::Semaphore()
 bool Semaphore::give()
 {
     take_count--;
-    if (pthread_mutex_unlock(&_lock) != 0) {
-        AP_HAL::panic("Bad semaphore usage");
-    }
     if (take_count == 0) {
         owner = (pthread_t)-1;
     }
+    if (pthread_mutex_unlock(&_lock) != 0) {
+        AP_HAL::panic("Bad semaphore usage");
+    }
     return true;
 }