reftable: increment pointer *after* reading uint16

This fixes timezone reads out of the reflog.

Change-Id: I126d8742e5e904a074c544514180720466164f7c
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java

@@ -624,6 +624,8 @@ public class ReftableTest {
 			assertEquals(ObjectId.zeroId(), lc.getReflogEntry().getOldId());
 			assertEquals(id(1), lc.getReflogEntry().getNewId());
 			assertEquals(who, lc.getReflogEntry().getWho());
+			// compare string too, to catch tz differences.
+			assertEquals(who.toExternalString(), lc.getReflogEntry().getWho().toExternalString());
 			assertEquals(msg, lc.getReflogEntry().getComment());
 
 			assertTrue(lc.next());

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/BlockReader.java

@@ -557,7 +557,9 @@ class BlockReader {
 	}
 
 	private short readInt16() {
-		return (short) NB.decodeUInt16(buf, ptr += 2);
+		short result =(short) NB.decodeUInt16(buf, ptr);
+		ptr += 2;
+		return result;
 	}
 
 	private int readVarint32() {