fix(blockStorage): Prevent invalid field caused panic

[wonchulee]

• Add to guard to prevent an invalid field used in the block storage.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0xc8 pc=0x102e8db9c]

goroutine 34 [running]:
github.com/terraform-providers/terraform-provider-ncloud/internal/service/server.createVpcBlockStorage(0x140005e6a00, 0x140006cc3c0)
	/Users/wonchul/work/ncloud/terraform/terraform-provider-ncloud/internal/service/server/block_storage.go:401 +0x8dc
github.com/terraform-providers/terraform-provider-ncloud/internal/service/server.createBlockStorage(0x1400007a4f0?, 0x102fd3325?)
	/Users/wonchul/work/ncloud/terraform/terraform-provider-ncloud/internal/service/server/block_storage.go:313 +0x24
github.com/terraform-providers/terraform-provider-ncloud/internal/service/server.resourceNcloudBlockStorageCreate(0x140005e6a00, {0x1032b58a0, 0x140006cc3c0})
	/Users/wonchul/work/ncloud/terraform/terraform-provider-ncloud/internal/service/server/block_storage.go:175 +0x70
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).create(0x10359a318?, {0x10359a318?, 0x14000155080?}, 0xd?, {0x1032b58a0?, 0x140006cc3c0?})
	/Users/wonchul/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:794 +0x130
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).Apply(0x140003dc1c0, {0x10359a318, 0x14000155080}, 0x140003528f0, 0x140005e6880, {0x1032b58a0, 0x140006cc3c0})
	/Users/wonchul/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:937 +0x874
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ApplyResourceChange(0x1400000da10, {0x10359a318?, 0x14000154f00?}, 0x140006363c0)
	/Users/wonchul/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/grpc_provider.go:1153 +0xaa4
github.com/hashicorp/terraform-plugin-mux/tf5to6server.v5tov6Server.ApplyResourceChange({{0x1035a6628?, 0x1400000da10?}}, {0x10359a318, 0x14000154f00}, 0x0?)
	/Users/wonchul/go/pkg/mod/github.com/hashicorp/[email protected]/tf5to6server/tf5to6server.go:38 +0x58
github.com/hashicorp/terraform-plugin-mux/tf6muxserver.(*muxServer).ApplyResourceChange(0x1400042a8c0, {0x10359a318?, 0x14000154c30?}, 0x14000636370)
	/Users/wonchul/go/pkg/mod/github.com/hashicorp/[email protected]/tf6muxserver/mux_server_ApplyResourceChange.go:36 +0x184
github.com/hashicorp/terraform-plugin-go/tfprotov6/tf6server.(*server).ApplyResourceChange(0x14000412140, {0x10359a318?, 0x14000154240?}, 0x14000264070)
	/Users/wonchul/go/pkg/mod/github.com/hashicorp/[email protected]/tfprotov6/tf6server/server.go:865 +0x2a8
github.com/hashicorp/terraform-plugin-go/tfprotov6/internal/tfplugin6._Provider_ApplyResourceChange_Handler({0x10353d0c0, 0x14000412140}, {0x10359a318, 0x14000154240}, 0x140005e6000, 0x0)
	/Users/wonchul/go/pkg/mod/github.com/hashicorp/[email protected]/tfprotov6/internal/tfplugin6/tfplugin6_grpc.pb.go:518 +0x1c0
google.golang.org/grpc.(*Server).processUnaryRPC(0x140001a4e00, {0x10359a318, 0x140001541b0}, {0x1035a2200, 0x14000430180}, 0x140001e8000, 0x140001bfec0, 0x103ef94d8, 0x0)
	/Users/wonchul/go/pkg/mod/google.golang.org/[email protected]/server.go:1369 +0xb58
google.golang.org/grpc.(*Server).handleStream(0x140001a4e00, {0x1035a2200, 0x14000430180}, 0x140001e8000)
	/Users/wonchul/go/pkg/mod/google.golang.org/[email protected]/server.go:1780 +0xb20
google.golang.org/grpc.(*Server).serveStreams.func2.1()
	/Users/wonchul/go/pkg/mod/google.golang.org/[email protected]/server.go:1019 +0x84
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 27
	/Users/wonchul/go/pkg/mod/google.golang.org/[email protected]/server.go:1030 +0x13c

Error: The terraform-provider-ncloud plugin crashed!

[youngmn]

LGTM 👍