HPCC-12674 RoxieReleaseRow() Avoid locking decrements when possible

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
That doesn't avoid the repeated test.  The point is to avoid the second if() to minimize the branch instructions.

What you need to do is replace both if conditions with a single condition which uses a comma expression:

```
if (ROWCOUNT(rowCount) == 0) ? (atomic_set(&header->count, rowCount), true) : ROWCOUNT(atomic_dec_and_read(&header->count)) == 0)
```

While easier for the compiler to optimize, it is a bit tricky to read.   Since it already seemed to optimize it I didn't think it was worth it.  
@richardkchapman would it be better to be explicit?