Fix an indexing error in MatrixFreeOps for IProductQuadKernel

Issue/feature addressed

There is an indexing error in the IProductQuadKernel matrix free kernel function that appear only when nm1 != nm0.

Proposed solution

This MR fixes the indexing error.

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

Notes

Please add any other information that could be useful for reviewers.

Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).