reftable: remove assert calls

The effect of assert is defined by compiler flags, so this code
introduced a potential vector for corruption. 

Change-Id: I12197432e4351a5bd4aa24d352a19937721845c3
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsReftableDatabase.java

@@ -163,7 +163,9 @@ public class DfsReftableDatabase extends DfsRefDatabase {
 	 *             if tables cannot be opened.
 	 */
 	protected DfsReftableStack stack() throws IOException {
-		assert getLock().isLocked();
+		if (!getLock().isLocked()) {
+			throw new IllegalStateException("most hold lock to access stack"); //$NON-NLS-1$
+		}
 		DfsObjDatabase odb = getRepository().getObjectDatabase();
 
 		if (ctx == null) {

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

@@ -138,7 +138,10 @@ public abstract class ReftableDatabase {
 	 *             on I/O problems
 	 */
 	private Reftable reader() throws IOException {
-		assert lock.isLocked();
+		if (!lock.isLocked()) {
+			throw new IllegalStateException(
+					"must hold lock to access merged table"); //$NON-NLS-1$
+		}
 		if (mergedTables == null) {
 			mergedTables = openMergedReftable();
 		}