fix for value may be null on this code path (found by FindBugs)

h2/src/tools/org/h2/dev/store/btree/Dump.java

@@ -109,12 +109,14 @@ public class Dump {
         } catch (IOException e) {
             writer.println("ERROR: " + e);
         } finally {
+            if (file != null) {
                 try {
                     file.close();
                 } catch (IOException e) {
                     // ignore
                 }
             }
+        }
         writer.println();
         writer.flush();
     }