json: actually use numeric C locale we just allocated
    
    This fixes formatting of JSON real values, and uses C locale for them.
    It's kinda interesting that this wasn't noticed before: the C locale
    object we allocated was not used, hence doing the dance had zero effect.
    
    This makes "test-varlink" pass again on systems with non-C locale.
    
    (My guess: noone noticed this because "long double" was used before by
    the JSON code and that had no locale supporting printer or so?)
    
    (cherry picked from commit 93258c7d72fae23c9f8103c98dd0e79a24838e26)