[ujson] Fixing seg faults in unit tests

- The following unit tests were causing segfaults:
```
test_doubleLongDecimalIssue
test_invalidDoublePrecision
test_doublePrecisionTest
test_encodeDecimal
test_doubleLongIssue
test_encodeDecodeLongDecimal
```
- I changed a constant that couldn't be `const` because it can be overwritten by
  a passed in parameter