Audio: Fix listener.update() and add regression test.

Any empty group would result in application crash.

Signed-off-by: Squareys <Squareys@googlemail.com>

src/Magnum/Audio/Listener.cpp

@@ -88,7 +88,7 @@ template<UnsignedInt dimensions> void Listener<dimensions>::update(std::initiali
 
     objects.push_back(this->object());
     for(PlayableGroup<dimensions>& group : groups) {
-        for(UnsignedInt i = 0; i < groups.size(); ++i) {
+        for(UnsignedInt i = 0; i < group.size(); ++i) {
             objects.push_back(group[i].object());
         }
     }

src/Magnum/Audio/Test/ListenerTest.cpp

@@ -89,6 +89,7 @@ void ListenerTest::testUpdateGroups() {
     Object3D sourceObject{&scene};
     Object3D object{&scene};
     PlayableGroup3D group;
+    PlayableGroup3D emptyGroup;
     Playable3D playable{sourceObject, &group};
     Listener3D listener{object};
 
@@ -97,7 +98,7 @@ void ListenerTest::testUpdateGroups() {
     object.translate(offset);
     sourceObject.translate(offset*13.0f);
 
-    listener.update({group});
+    listener.update({group, emptyGroup});
 
     CORRADE_COMPARE(Renderer::listenerPosition(), offset);
     constexpr Vector3 rotatedFwd{-1.0f, 0.0f, 0.0f};