ASSERT handlecheck.c:602 running notepad on win10-1607 w/o any syscall info

This is at exit of:

```
# bin/drmemory -debug -verbose 3 -no_use_syscall_tables -ignore_kernel -dr_debug -dr_ops "-checklevel 0" -batch -- notepad
```

On win10-1607 x64 kernel.

That's w/o a syscall file, so this is testing unknown syscall handling and
is likely related to that.

```
~~Dr.M~~ ASSERT FAILURE (thread 6156): D:\derek\drmemory\git\src\drmemory\handlecheck.c:602: hashtable_lookup(&user_handle_table, (void *)entry->HandleValue) == NULL (kernel handle in user handle table)
```