Refactor NUL magic char

Author: garydgregory

src/main/java/org/apache/commons/compress/archivers/cpio/CpioArchiveOutputStream.java

@@ -68,6 +68,11 @@
  */
 public class CpioArchiveOutputStream extends ArchiveOutputStream<CpioArchiveEntry> implements CpioConstants {
 
+    /**
+     * The NUL character.
+     */
+    private static final char NUL = '\0';
+
     private CpioArchiveEntry entry;
 
     /**
@@ -379,7 +384,7 @@ private void writeBinaryLong(final long number, final int length, final boolean
      */
     private void writeCString(final byte[] str) throws IOException {
         out.write(str);
-        out.write('\0');
+        out.write(NUL);
         count(str.length + 1);
     }