loadbalancer_sg: nil-check subnet.Extract result

[SAY-5]

What this PR does / why we need it:

subnets.Get(...).Extract() can return a nil Subnet with no error in some OpenStack misconfigurations. Dereferencing subnet.CIDR then SIGSEGVs the OCCM service worker into CrashLoopBackOff. This adds a nil check that treats the nil result as not-found and continues, matching the existing 404 path.

Which issue this PR fixes(if applicable):
fixes #3101

Special notes for reviewers:
The Subnet field on the gophercloud GetResult is a pointer and can be nil even when Extract() returns no error: https://github.com/gophercloud/gophercloud/blob/f4ea62fe7ab75f55496aeaad98a0859170b546eb/openstack/networking/v2/subnets/results.go#L17-L21

Release note:

[openstack-cloud-controller-manager] Fix nil-pointer panic in loadbalancer security-group reconciler when subnets.Get returns a nil Subnet with no error.

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[stephenfin]

The fix seems reasonable given the Subnet field is a pointer and potentially nil.

However, you need to fix the commit message and add the missing release note section to the PR summary before we can test and merge this.

[kayrus]

IMHO this check needs to be implemented in gophercloud

[stephenfin]

Good point

[SAY-5]

Updated commit message + added release note. Thanks @stephenfin!