I'm surprised by nonchalance about bad inputs evoking bad program behavior. That attitude may have been excusable 50 years ago. By now, though, we have seen so much malicious exploitation of open avenues of "undefined behavior" that we can no longer ignore bugs that "can't happen when using the tool correctly". Mature software should not brook incorrect usage.
"Bailing out near line 1" is a sign of defensive precautions. Crashes and unjustified output betray their absence.
I commend attention to the LangSec movement, which advocates for rigorously enforced separation between legal and illegal inputs.