SECRETS: Return a nicer error message on request with no PUT data
    
    I managed to create this pathological situation with the tcurl tool
    which didn't send any PUT data. The error in sssd-secrets was quite
    strange (ENOMEM). This patch just adds a safeguard sooner so that we
    return a graceful error.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>