Fix: Two reject statements in the same scope causes C++ compilation to fail

[WardBrian]

This would be useless code, so it is relatively benign, but it still shouldn't cause a compile failure.

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Fixed an issue where multiple reject statements could cause the C++ compiler to fail

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8f2dbe7) 89.88% compared to head (771f372) 89.88%.
Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1396   +/-   ##
=======================================
  Coverage   89.88%   89.88%           
=======================================
  Files          63       63           
  Lines       10505    10506    +1     
=======================================
+ Hits         9442     9443    +1     
  Misses       1063     1063           

Files	Coverage Δ
src/frontend/Typechecker.ml	91.97% <100.00%> (ø)
src/stan_math_backend/Lower_stmt.ml	95.38% <100.00%> (+0.02%)	⬆️

[nhuurre]

LGTM