pkg/extend fix panic for empty partition tables

[ChrisIgel]

- What I did

Fix a go panic when trying to resize a device with a partition table with 0 partitions.

- How I did it

Check the condition and return early.

- How to verify it

Create and mount a dummy device with an empty partition table and let pkg/extend try to resize the device.
The following error should no longer occur:

panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
main.extend({0xc000013880, 0xa}, {0x5341a3, 0x4})
        /<redacted>/linuxkit/pkg/extend/extend.go:93 +0x119c
main.autoextend({0x5341a3, 0x4})
        /<redacted>/linuxkit/pkg/extend/extend.go:60 +0x1b0
main.main()
        /<redacted>/linuxkit/pkg/extend/extend.go:333 +0x8e

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[deitch]

This is fine, can you run ./scripts/update-component-sha.sh --pkg ./pkg/extend from within the linuxkit repo root, so the examples and everything pick it up?

[ChrisIgel]

This is fine, can you run ./scripts/update-component-sha.sh --pkg ./pkg/extend from within the linuxkit repo root, so the examples and everything pick it up?

I already did that :)

[deitch]

Yeah, you are right. I was sure it was elsewhere, but it is not.