Fix DB meta lost bug 2

[kangkaisen]

For #2167

I didn't notice CHECKPOINT catalog will destroy after every saveImage.

finally {
            // destroy checkpoint catalog, reclaim memory
            catalog = null;
            Catalog.destroyCheckpoint(); 
        }

So the following check is meaningless, fullNameToDb always don't contain InfoSchemaDb name.

if (!fullNameToDb.containsKey(dbName)) {

So I use real Catalog instance fullNameToDb to do this check.

[morningman]

LGTM