fixes #2662

[Sintun]

fixes #2662

This PR fixes the usage of uninitialized memory that can lead to throws and segfaults.

I think the original author wanted to add ones to a zero-initialized Mat object.
I could have used another Constructor for the Mat object, but i chose to make the minimal amount of changes to the code to work as intended.

My proposed change does not change the used algorithm.

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  Patch to opencv_extra has the same branch name.
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[alalek]

Thank you for contribution!