Call va_end after va_copy in json_vsprintf
Call va_end after va_copy in json_vsprintf

As said in man doc:
  "Each  invocation  of va_copy() must be matched by a corresponding
   invocation of va_end() in the same function."

va_copy may alloc memory in some system, it's necessay to free it by
va_end.

Fixes: efe6c7b3f2b3 ("Add json_sprintf and json_vsprintf")
Signed-off-by: Xin Long <ed9bd6f22d2a1e0b1507d3ddc300dc248265d19a@gmail.com>

Mephistophiles/jansson,Mephistophiles/jansson,markalanj/jansson,markalanj/jansson,akheron/jansson,akheron/jansson,markalanj/jansson,AmesianX/jansson,bstarynk/jansson,bstarynk/jansson,AmesianX/jansson,akheron/jansson,bstarynk/jansson