Fixing NaN endPoint Issue Causing Extra Lines in Marker Drawing

[tugcearar]

This pull request aims to resolve an issue where extra lines appear on the screen during the drawing process with marker. The problem stems from situations where the endPoint value becomes NaN. To mitigate this, the proposed solution involves implementing a check for isNaN before processing the endPoint.

Changes Made: Introduced a conditional check for NaN values in the endPoint. Added validation to ensure that only valid numerical values are used for drawing, preventing unexpected artifacts.

Screenshot and video:

https://github.com/tumblr/kanvas-ios/assets/12747384/03ef4df6-7a90-4c4d-b6ac-0f7ad1787355

Thank you for reviewing this pull request. Your feedback is essential in ensuring the quality and functionality of the codebase.