AIOOB in ChangeIdUtil.indexOfChangeId if amended message is a newline

Bug: 456792
Change-Id: Idf38a0db7dc72d672ff185dcac2f41f0012a6673
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

org.eclipse.jgit.test/tst/org/eclipse/jgit/util/ChangeIdUtilTest.java

@@ -639,6 +639,10 @@ public class ChangeIdUtilTest {
 
 	@Test
 	public void testIndexOfChangeId() {
+		assertEquals(-1, ChangeIdUtil.indexOfChangeId("", "\n"));
+		assertEquals(-1, ChangeIdUtil.indexOfChangeId("\n", "\n"));
+		assertEquals(-1, ChangeIdUtil.indexOfChangeId("\r\n", "\r\n"));
+
 		assertEquals(3, ChangeIdUtil.indexOfChangeId("x\n" + "\n"
 				+ "Change-Id: I3b7e4e16b503ce00f07ba6ad01d97a356dad7701\n",
 				"\n"));

org.eclipse.jgit/src/org/eclipse/jgit/util/ChangeIdUtil.java

@@ -229,6 +229,8 @@ public class ChangeIdUtil {
 	 */
 	public static int indexOfChangeId(String message, String delimiter) {
 		String[] lines = message.split(delimiter);
+		if (lines.length == 0)
+			return -1;
 		int indexOfChangeIdLine = 0;
 		boolean inFooter = false;
 		for (int i = lines.length - 1; i >= 0; --i) {