mpl_nested_loop.hh: minor code-style changes

2 jobs for master
Status Job ID Name Coverage
  Test
failed #4595
amd64
job_clang_amd64

failed #4594
amd64
job_gcc_amd64

 
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failed
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failed
job_clang_amd64 Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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