Fix ValueError with invalid field name in format.

```
  File "/Users/skumaran/github/experiments/sphinxextension/exampleproject/hidden_code_block.py", line 104, in visit_hcb_html
    fill_header = {'divname': 'hiddencodeblock{}'.format(HCB_COUNTER),
ValueError: zero length field name in format
```
Merge pull request #1 from orsenthil/patch-1

Fix ValueError with invalid field name in format.