MINOR: sample: always set a new sample's owner before evaluating it

MINOR: sample: always set a new sample's owner before evaluating it

Some functions like sample_conv_var2smp(), var_get_byname(), and
var_set_byname() directly or indirectly need to access the current
stream and/or session and must find it in the sample itself and not
as a distinct argument. Thus we first need to call smp_set_owner()
prior to each such calls.