fixing crash on shutdown hook #1029 (#1032)

* fixing the crash on shutdown introduced in #910 by changing the command that gets run on on deleteOnExit 
Files.delete() -> Files.deleteIfExists() 
this will prevent the jvm from crashing on shutdown if the temporary files get cleaned up before it closes

* This still leaves open the possibilty of crash on shutdown if the temporary file is unable to be removed for some other reason.  That's subtly different than the File version of deleteOnExit, but it may be useful to get these error messages in the unusual case of failing to delete a file that exists.

* fixes #1029

src/main/java/htsjdk/samtools/util/IOUtil.java

@@ -360,16 +360,16 @@ public static void deleteOnExit(final Path path) {
      *
      * @see #deleteOnExit(Path)
      */
-    protected static final class DeletePathThread extends Thread {
+    static final class DeletePathThread extends Thread {
 
         private final Path path;
 
-        protected DeletePathThread(Path path) {this.path = path;}
+        DeletePathThread(Path path) {this.path = path;}
 
         @Override
         public void run() {
             try {
-                Files.delete(path);
+                Files.deleteIfExists(path);
             } catch (IOException e) {
                 throw new RuntimeIOException(e);
             }