Skip broken symbolic ref in DfsReftableDatabase

We skipped the broken symbolic reference in other implementation like
DfsRefDatabase, RefDirectory. The broken symbolic reference may cause
NPE when caller forget to have a null check against the object id before
calling parse it.

Change-Id: If5e07202e9ee329d0bd9488936d79c98143c7ad9
Signed-off-by: Zhen Chen <czhen@google.com>

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

@@ -241,7 +241,7 @@ public class DfsReftableDatabase extends DfsRefDatabase {
 					: table.seekRef(prefix)) {
 				while (rc.next()) {
 					Ref ref = table.resolve(rc.getRef());
-					if (ref != null) {
+					if (ref != null && ref.getObjectId() != null) {
 						all.add(ref);
 					}
 				}