session_driver: avoid division by zero, catch API violation

session_driver: avoid division by zero, catch API violation

Avoid a division by zero, by not using a zero unitsize in a modulo
operation. As a byproduct, avoid processing and counting input that
neither has analog nor logic data. This should never have happened,
but the change now catches the error if invalid input is seen.

This was reported by clang's scan-build.