Fix unchecked optional access in compile subcommand

[ottmar-zittlau]

Hi,

I fixed a small issue that I found inside the "compile subcommand" component:
The program can be crashed by running bazel run -- toolchain:carbon compile --dump-mem-usage "non-existing-file.carbon" - i.e. by activating the memory usage dump flag and passing a non-existing file.

Best regards,
oz

[chandlerc]

Fix looks great -- would you be up for adding a test case?

This seems like a thing effectively tested with unittests here: https://github.com/carbon-language/carbon-lang/blob/trunk/toolchain/driver/driver_test.cpp

[ottmar-zittlau]

Yes - I added a test for this specific flag only.
I thought about repeating the test also for the other flags, but didn't want to stretch this patch unnecessarily.
Let me know, if you think this would make sense.