solver: stub out sysSampler close

Follow-up to ce332e1 - this didn't
resolve the access to sysSampler.Close, we need to stub this one out as
well!

Signed-off-by: Justin Chadwell <me@jedevc.com>
(cherry picked from commit d0b170c)

Author: jedevc

solver/llbsolver/solver.go

@@ -125,8 +125,10 @@ func New(opt Opt) (*Solver, error) {
 
 func (s *Solver) Close() error {
 	s.solver.Close()
-	err := s.sysSampler.Close()
-	return err
+	if s.sysSampler != nil {
+		return s.sysSampler.Close()
+	}
+	return nil
 }
 
 func (s *Solver) resolver() solver.ResolveOpFunc {