update.go: fix nil pointer risk in Intel RDT initialization

Static analysis revealed a possible nil dereference when applying
Intel RDT configurations. The `NewManager()` function can return
nil if Intel RDT is not supported by the kernel (e.g. no resctrl
filesystem mounted), but the caller in `update.go` did not check
for this condition.

This patch adds an explicit nil check after NewManager() is called,
with a descriptive log message to help users understand the failure.
In addition, `NewManager()` itself now logs the underlying reason
(using the error from `Root()`), improving observability.

This issue was originally identified via static analysis:
> update.go:367:9: undefined: log
> and: Return value of function 'NewManager' will be overwritten

Signed-off-by: AntonMoryakov <[email protected]>

Author: AntonMoryakov

libcontainer/intelrdt/intelrdt.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/opencontainers/cgroups/fscommon"
 	"github.com/opencontainers/runc/libcontainer/configs"
+	"github.com/sirupsen/logrus"
 )
 
 /*
@@ -161,6 +162,7 @@ func NewManager(config *configs.Config, id string, path string) *Manager {
 	}
 	if _, err := Root(); err != nil {
 		// Intel RDT is not available.
+		logrus.Errorf("Intel RDT is not available: %v", err)
 		return nil
 	}
 	return newManager(config, id, path)

update.go

@@ -363,6 +363,10 @@ other options are ignored.
 				}
 				config.IntelRdt = &configs.IntelRdt{}
 				intelRdtManager := intelrdt.NewManager(&config, container.ID(), state.IntelRdtPath)
+				if intelRdtManager == nil {
+					logrus.Errorf("Intel RDT manager creation failed: likely due to missing resctrl filesystem")
+					return nil
+				}
 				if err := intelRdtManager.Apply(state.InitProcessPid); err != nil {
 					return err
 				}