create_j1939db-json.py: handle variable-length SPNs with no delimiter in a multi-SPN PGN

[BenGardiner]

there are several in (at least) the 201611 DA e.g.

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Warning: removing SPN 987 from PGN 64862 because it is variable-length with no delimiter in a multi-SPN PGN. This likely an under-specification in the DA.
Warning: removing SPN 624 from PGN 64862 because it is variable-length with no delimiter in a multi-SPN PGN. This likely an under-specification in the DA.
Warning: removing SPN 623 from PGN 64862 because it is variable-length with no delimiter in a multi-SPN PGN. This likely an under-specification in the DA.
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