#2 add_nsc_numbers adds even if available

[Egon Willighagen]

The program bin/cas/add_nsc_number adds the NSC number
even when it is already available:

Processing ../../data/wml/123/4nitrobenzylalcohol.xml.
NSC number added...

the file becomes:

<INDEX CLASS="CAS-NUMBER">619-73-8</INDEX>
<INDEX CLASS="NSC-NUMBER">5389</INDEX>
<INDEX
CLASS="SMILES">[N+]([O-])(=O)C1=CC=C(CO)C=C1</INDEX>
<INDEX CLASS="NSC-NUMBER">5389</INDEX>