Add missing checks against null.

The program can dereference a null pointer because it does not check the return value of a function that might return
null.

src/main/java/com/threerings/getdown/classpath/cache/GarbageCollector.java

@@ -30,8 +30,11 @@ public class GarbageCollector
                 }
 
                 File folder = file.getParentFile();
-                if (folder.list().length == 0) {
-                    FileUtil.deleteHarder(folder);
+                if (folder != null) {
+                    String[] children = folder.list();
+                    if (children != null && children.length == 0) {
+                        FileUtil.deleteHarder(folder);
+                    }
                 }
             }
         });

src/main/java/com/threerings/getdown/util/FileUtil.java

@@ -187,14 +187,20 @@ public class FileUtil
      */
     public static void walkTree (File root, Visitor visitor)
     {
-        Deque<File> stack = new ArrayDeque<>(Arrays.asList(root.listFiles()));
-        while (!stack.isEmpty()) {
-            File currentFile = stack.pop();
-            if (currentFile.exists()) {
-                visitor.visit(currentFile);
-                if (currentFile.isDirectory()) {
-                    for (File file: currentFile.listFiles()) {
-                        stack.push(file);
+        File[] children = root.listFiles();
+        if (children != null) {
+            Deque<File> stack = new ArrayDeque<>(Arrays.asList(children));
+            while (!stack.isEmpty()) {
+                File currentFile = stack.pop();
+                if (currentFile.exists()) {
+                    visitor.visit(currentFile);
+                    if (currentFile.isDirectory()) {
+                        File[] currentChildren = currentFile.listFiles();
+                        if (currentChildren != null) {
+                            for (File file : currentChildren) {
+                                stack.push(file);
+                            }
+                        }
                     }
                 }
             }