Fix sanitizeErrorMessage corner case

[pscanf]

(I believe this might be the cause of webdriverio/webdriverio#1726)

Scenario:

• a failing test throws an error
• the error gets caught and passed to sanitizeErrorMessage to fix its stack trace
• the error stack is only two-lines long, therefore the stack array has length 2
• at least two elements are removed by the stack array (since it's shift-ed twice)
• the stack array is now empty, and lineToFix is undefined, which causes a TypeError: Cannot read property 'indexOf' of undefined

(I can reproduce this in a client project I'm working on that I can't share, let me know if an "open reproduction" is needed)

The error is a bit erratic. On my local machine (macOS), with the exact same run, sometimes it happens, sometimes it doesn't, meaning sometimes the stack trace has 2 lines, sometimes 3. In a docker container (ubuntu) it appears to be systematic. I did not succeed to pinpoint the reason behind that.

This pull request attempts to fix the error described above by adding an undefined-check on lineToFix. Now when lineToFix === undefined the else case will be executed. I'm not sure if that is the correct/desired fallback though.

[christian-bromann]

Undefined checks are always appreciated 👍

[pscanf]

Thanks. :)

[monolithed]

@pscanf, more thanks for that! @christian-bromann, could you make a release, please?