groot/rtree: fix handling of rleaf-element with a "This" StreamerSTL

[sbinet]

Fixes #736.

[codecov]

Codecov Report

Merging #739 into master will decrease coverage by 0.00%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #739      +/-   ##
==========================================
- Coverage   71.50%   71.49%   -0.01%     
==========================================
  Files         384      384              
  Lines       48908    48912       +4     
==========================================
  Hits        34972    34972              
- Misses      11681    11684       +3     
- Partials     2255     2256       +1     

Impacted Files	Coverage Δ
groot/rtree/rleaf.go	79.31% <20.00%> (-2.25%)	⬇️
groot/rtree/rvar.go	96.42% <100.00%> (ø)

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