Fix crash when using --tree on musl

[leso-kn]

This PR addresses the crash described in #1549 and adds the value passed to the --tree argument to cfg.rocks_trees.
Tested and working on alpine:edge running luarocks-5.1 v3.9.2.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4cfcf9d) 79.10% compared to head (afdf03f) 78.97%.

❗ Current head afdf03f differs from pull request most recent head 7ac3a6f. Consider uploading reports for the commit 7ac3a6f to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1550      +/-   ##
==========================================
- Coverage   79.10%   78.97%   -0.13%     
==========================================
  Files          89       89              
  Lines       11092    11088       -4     
==========================================
- Hits         8774     8757      -17     
- Misses       2318     2331      +13     

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[hishamhm]

The combination of all possible configurations and flags added over the year to tend for numerous scenarios and use-cases often end up leading to some surprising behaviors... A big cleanup of the tree management code would do it good, but given backward compatibility constraints, I prefer keeping changes to this logic minimal for now.

Since that you reported this fixes a crash and the test suite seems to happy about it, I am going to merge your change, thanks!