fix issue #5461 (segfault after removing redundant materials)

[stephengold]

Update mNumMaterials before early return from RemoveRedundantMatsProcess::Execute() .

[stephengold]

I tested this fix on the "Running.gltf" model from issue #5461 and verified that the command completes successfully: no segfault.
I believe this pull request solves #5461 (without introducing any new issues) and is ready to be integrated.

[kimkulling]

During the review I have detected 2 leaks.

[stephengold]

@kimkulling Thanks for solving the memory leaks.

[kimkulling]

Hm, tests are failing for g++ on Ubuntu. I cannot reproduce it on windows. I will try to analyze it with Linux.

[stephengold]

I tried to reproduce the test failure on my Linux desktop and was unsuccessful: all 574 tests passed.
My desktop used GNU 11.4.0, just like the GHA runner.

[stephengold]

It seems my merge solved the test failure!
I believe this PR is now ready for integration.

[stephengold]

After another merge from 'master', the tests are failing again.

[stephengold]

Once again, merging has solved the failing checks. Please integrate this PR.

[stephengold]

After another merge from 'master', another failing check.
I see no alternative except to wait for another merge opportunity.

[stephengold]

Once again, merging has solved the failing checks. @kimkulling please integrate this PR.

[kimkulling]

Updated again, let's hope that the tests are green.

[kimkulling]

Merged, thanks a lot for your contribution.