ArchiveTest: Check result of BufferedInputStream#skip

Change-Id: I5168c75ca2dda6d1e8c8a6c29043ac781fe9295a
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/ArchiveTest.java

 package org.eclipse.jgit.pgm;
 
 import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 import static org.junit.Assume.assumeNoException;
 
 		BufferedInputStream in = new BufferedInputStream(
 				new FileInputStream(file));
 		try {
-			in.skip(offset);
+			if (offset > 0) {
+				long skipped = in.skip(offset);
+				assertEquals(offset, skipped);
+			}
 
 			byte[] actual = new byte[magicBytes.length];
 			in.read(actual);