CLEANUP: vars: always pre-initialize smp in vars_parse_cli_get_var()

CLEANUP: vars: always pre-initialize smp in vars_parse_cli_get_var()

In issue #1200 Coverity believes we may use an uninitialized field
smp.sess here while it's not possible because the returned variable
necessarily matches SCOPE_PROC hence smp.sess is not used. But it
cannot see this and it could be confusing if the code later evolved
into something more complex. That's not a critical path so let's
first reset the sample.