fix deadlock in queue drop

ethcore/src/verification/queue/mod.rs

@@ -681,8 +681,12 @@ impl<K: Kind> Drop for VerificationQueue<K> {
 		*self.state.0.lock() = State::Exit;
 		self.state.1.notify_all();
 
-		// wake up all threads waiting for more work.
-		self.more_to_verify.notify_all();
+		// acquire this lock to force threads to reach the waiting point
+		// if they're in-between the exit check and the more_to_verify wait.
+		{
+			let _more = self.verification.more_to_verify.lock().unwrap();
+			self.more_to_verify.notify_all();
+		}
 
 		// wait for all verifier threads to join.
 		for thread in self.verifier_handles.drain(..) {