fixes #1907: skip MGLPolyline/MGLPolygons with zero points

platform/ios/MGLMapView.mm

@@ -1828,6 +1828,10 @@ - (void)addAnnotations:(NS_ARRAY_OF(id <MGLAnnotation>) *)annotations
 
         if ([annotation isKindOfClass:[MGLMultiPoint class]])
         {
+            NSUInteger count = [(MGLMultiPoint *)annotation pointCount];
+
+            if (count == 0) break;
+
             CGFloat alpha = (delegateImplementsAlphaForShape ?
                                 [self.delegate mapView:self alphaForShapeAnnotation:annotation] :
                                 1.0);
@@ -1881,8 +1885,6 @@ - (void)addAnnotations:(NS_ARRAY_OF(id <MGLAnnotation>) *)annotations
                                        userInfo:nil] raise];
             }
 
-            NSUInteger count = [(MGLMultiPoint *)annotation pointCount];
-
             CLLocationCoordinate2D *coordinates = (CLLocationCoordinate2D *)malloc(count * sizeof(CLLocationCoordinate2D));
             [(MGLMultiPoint *)annotation getCoordinates:coordinates range:NSMakeRange(0, count)];