fix(codegen): crash on missing nested private function

[Wodann]

This is a fix for the Mun v0.2 release, which should also be cherry-picked/pushed to the release/v0.2 branch and released as v0.2.1 (at some point).

We've also discussed a more elaborate solution where we first do a pass to determine all "used" functions. I'll leave that for future work, as this patch should be rolled out

[codecov]

Codecov Report

Merging #221 into master will increase coverage by 0.00%.
The diff coverage is 88.88%.

@@           Coverage Diff           @@
##           master     #221   +/-   ##
=======================================
  Coverage   81.10%   81.10%           
=======================================
  Files         182      182           
  Lines       11903    11911    +8     
=======================================
+ Hits         9654     9661    +7     
- Misses       2249     2250    +1     

Impacted Files	Coverage Δ
crates/mun_codegen/src/ir/dispatch_table.rs	84.42% <83.33%> (-0.19%)	⬇️
crates/mun_codegen/src/test.rs	98.19% <100.00%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 73d0839...6049096. Read the comment docs.