control: make sure sending nil cache options does not panic

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

control/control.go

@@ -376,6 +376,9 @@ func (c *Controller) Solve(ctx context.Context, req *controlapi.SolveRequest) (*
 	atomic.AddInt64(&c.buildCount, 1)
 	defer atomic.AddInt64(&c.buildCount, -1)
 
+	if req.Cache == nil {
+		req.Cache = &controlapi.CacheOptions{} // make sure cache options are initialized
+	}
 	translateLegacySolveRequest(req)
 
 	defer func() {