Fixed save_imatrix to match old behaviour for MoE (#7099)

* Fixed save_imatrix to match old behaviour for MoE

This fix is simple and clear, but unnecessarily doubles the memory overhead..

* Fixed missing idx variable

* Unconditionally increment ncall

Co-authored-by: slaren <slarengh@gmail.com>

* Fixed 2 bugs in save_imatrix()

- Fixed segfault bug because the counts vector needed to be created.
- Fixed pre-existing bug didn't actually add to the counts for "--combine" option.

* ncall needs summing too

* Trailing whitespace

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Co-authored-by: slaren <slarengh@gmail.com>