src: don't crash on invalid script positions

[kvakil]

This "fixes" the segfault reported in #422, in the sense that you no longer get a segfault. However the printing does not actually work, i.e. you currently get an error like this:

(lldb) v8 i -s 0x2196b1a09a29
error: Invalid source range, start_pos=3108, len=-3098, source_len=10

I'm deeming this better than segfaulting. We should really never be segfaulting as the coredump might be incomplete/partially corrupted. (Also, we already know function printing on v16 doesn't work right now.)

[codecov-commenter]

Codecov Report

Base: 73.65% // Head: 73.59% // Decreases project coverage by -0.05% :warning:

Coverage data is based on head (4a950e7) compared to base (eb969e1). Patch coverage: 40.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #426 +/- ## ========================================== - Coverage 73.65% 73.59% -0.06% ========================================== Files 34 34 Lines 4995 4999 +4 ========================================== Hits 3679 3679 - Misses 1316 1320 +4 ``` | [Impacted Files](https://codecov.io/gh/nodejs/llnode/pull/426?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/llv8.cc](https://codecov.io/gh/nodejs/llnode/pull/426/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2xsdjguY2M=) | `71.25% <40.00%> (-0.38%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[tony-go]

Thanks ^^

Definitely better than segfault. 😃

But we don't really know why that is happening. I'm correct?

[No9]

LGTM to so merging