Fixed compiler and valgrind warnings

Added missing DBUG_xxx_RETURN statements
Fixed some usage of not initialized variables (as found by valgrind)
Ensure that we don't remove locked tables used as name locks from open table cache until unlock_table_names() are called.
This was fixed by having drop_locked_name() returning any table used as a name lock so that we can free it in unlock_table_names()
This will allow Tomas to continue with his work to use namelocks to syncronize things.

Note: valgrind still produces a lot of warnings about using not initialized code and shows memory loss errors when running the ndb tests

sql/handler.h

@@ -686,6 +686,8 @@ public:
   {
     do_prelock(tables, count);
   }
+  virtual ~TABLEOP_HOOKS() {}
+  TABLEOP_HOOKS() {}
 
 private:
   /* Function primitive that is called prior to locking tables */