Fix crash in MemoryPool

[InflexCZE]

When range-iterating MemoryPool, when there are exactly N x SIZE records, the iterator will fail to stop at last block and continue reading from nullptr chunk.

This typically crashes at SaveCapture -> DumpFrames -> DumpSummary when reading frames.
https://github.com/bombomby/optick/blob/master/src/optick_core.cpp#L1134

Simple reproduction

Optick::StartCapture();
for (128)
{
   RecordFrame();
}
Optick::SaveCapture();