assertions added to help with #1201 investigation

h2/src/main/org/h2/mvstore/MVStore.java

@@ -505,9 +505,12 @@ public class MVStore implements AutoCloseable {
         M map;
         if (id >= 0) {
             map = openMap(id, builder);
+            assert builder.getKeyType() == null || map.getKeyType().getClass().equals(builder.getKeyType().getClass());
+            assert builder.getValueType() == null || map.getValueType().getClass().equals(builder.getValueType().getClass());
         } else {
             HashMap<String, Object> c = new HashMap<>();
             id = lastMapId.incrementAndGet();
+            assert getMap(id) == null;
             c.put("id", id);
             c.put("createVersion", currentVersion);
             map = builder.create(this, c);