Fix nullptr exception (#1355)

* Add test cases to url naming

* Fix possible nullptr exception when parent does not exist

Author: tdrwenski

cdm/core/src/main/java/thredds/filesystem/MFileOS.java

@@ -88,9 +88,11 @@ public String getName() {
     return file.getName();
   }
 
+  @Nullable
   @Override
   public MFile getParent() {
-    return new MFileOS(file.getParentFile());
+    File parent = file.getParentFile();
+    return parent == null ? null : new MFileOS(parent);
   }
 
   @Override

cdm/core/src/test/java/ucar/nc2/util/TestURLnaming.java

@@ -5,6 +5,8 @@
 
 package ucar.nc2.util;
 
+import static com.google.common.truth.Truth.assertThat;
+
 import java.lang.invoke.MethodHandles;
 import org.junit.Test;
 import org.slf4j.Logger;
@@ -30,6 +32,14 @@ public void testResolve() {
     testResolve("file://test/me/", "file:/wanna", "file:/wanna");
     testResolve("file://test/me/", "C:/wanna", "C:/wanna");
     testResolve("http://test/me/", "file:wanna", "file:wanna");
+
+    testResolve("urlWithoutSlash", "file:///path/with/slash", "file:///path/with/slash");
+  }
+
+  @Test
+  public void testResolveFile() {
+    assertThat(URLnaming.resolveFile("urlWithoutSlash", "file:///path/with/slash"))
+        .isEqualTo("file:///path/with/slash");
   }
 
   private void testResolve(String base, String rel, String result) {