fix JSON upload in rlm_rest

Previously the POSTED json would be invalid - it would truncate at the
end of the first tuple (because of accidentally including a \0 in the
string).  This change ensures that the returned length from
vp_prints_value_json does not include the nul terminator (which is
conventional and I assme the intent)
Merge pull request #439 from Xeralux/fixrest2

fix JSON upload in rlm_rest