Avoid crash with a lock file

[kaneshin]

Possibly crash this code.

[codecov-io]

Codecov Report

Merging #162 into master will increase coverage by 0.09%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
+ Coverage   78.88%   78.98%   +0.09%     
==========================================
  Files          24       23       -1     
  Lines        3709     3654      -55     
==========================================
- Hits         2926     2886      -40     
+ Misses        582      569      -13     
+ Partials      201      199       -2

Impacted Files	Coverage Δ
source_manager.go	91.41% <100%> (+0.03%)	✅
analysis.go	85.29% <ø> (-0.01%)	❌
cmd.go
trace.go	73.19% <ø> (+0.97%)	✅

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[sdboyer]

I appreciate the PR, but this is intentional. Assuming the calling code has created the *SourceMgr through NewSourceManager() and is not messing with unsafe, it's not possible to reach that point in the logic and have that file handle be nil.

Or at least, it shouldn't be possible. If it does happen, it indicates the code has followed a significant different path than I thought possible, some correctness concerns may have been violated, and there is no knowable way to recover, and no way of communicating this back to the calling code, so a panic is merited. Silently letting it go would not be great.

However, being that this is intentional, I'd accept a version of this PR where we explicitly panic with an informative message if it is nil.

[sdboyer]

If you're willing to do that, please also add a test case for it - that really codifies that it's intentional.