Fix unbounded write of sscanf()

[szsam]

Buffer write operations that do not control the length of data written may overflow.

[Machiry]

Hello @symisc and @timgates42 ,

My name is Aravind Machiry, Assistant Professor at Purdue's ECE Department.

Thank you for considering this pull request. This pull request was the result of our on-going research work (along with @szsam) to improve the security and quality of open-source embedded projects.

In addition to scanning codebases with CodeQL, we are also doing a short (~4 minutes) survey to understand the use of static analysis tools like gcc -Wall and CodeQL in embedded software projects.

It would greatly benefit our research if you could fill this anonymous survey: https://purdue.ca1.qualtrics.com/jfe/form/SV_0OnXfr5plPe1QCa

Thank you,
Aravind