Fix for a divide by zero related crash

Causes output that will crash the block explorer, e.g. check https://bkchain.org/ppc/block/0000000032fe677166d54963b62a4677d8957e87c508eaa4fd7eb1c880cd27e3
```
{"id":1,"jsonrpc":"2.0","result":{"avg_coinage":nan, .. rest left out
```

JSON.parse is not able to correctly parse 'nan' as anything useful. Fix it by setting the coin_age to zero if the divisor is non-positive.