Fix NPE for `OsmReader#estimateMemoryUsageBytes()` (#389)

A `NullPointerException` was thrown when this method was called after
calling `close()` (which happens when using the `PrometheusStats`
client).

planetiler-core/src/main/java/com/onthegomap/planetiler/reader/osm/OsmReader.java

@@ -541,7 +541,7 @@ public class OsmReader implements Closeable, MemoryEstimator.HasEstimate {
   @Override
   public long estimateMemoryUsageBytes() {
     long size = 0;
-    size += waysInMultipolygon.serializedSizeInBytes();
+    size += waysInMultipolygon == null ? 0 : waysInMultipolygon.serializedSizeInBytes();
     // multipolygonWayGeometries is reported separately
     size += estimateSize(wayToRelations);
     size += estimateSize(relationInfo);