core: always release Irq_proxy startup lock

Prior this patch the startup lock was not released if the call of
'_associate()' failed. In this condition, the caller of the constructor
was infinitely blocked.

base/src/core/include/irq_proxy.h

@@ -154,10 +154,12 @@ class Genode::Irq_proxy : public THREAD,
 		 */
 		void entry()
 		{
-			if (_associate()) {
-				_startup_lock.unlock();
+			bool const associate_suceeded = _associate();
+
+			_startup_lock.unlock();
+
+			if (associate_suceeded)
 				_loop();
-			}
 		}
 
 		/**