Treat nil value on SecurityContext.ReadOnlyRootFilesystem as false

[welteki]

Description

Add nil check when dereferencing SecurityContext.ReadOnlyRootFilesystem and
treat nil value on SecurityContext.ReadOnlyRootFilesystem as false.

Motivation and Context

• I have raised an issue to propose this change (required)
• My issue has received approval from the maintainers or lead with the design/approved label

This should fix a nil pointer dereference reported by a customer.

How Has This Been Tested?

I was not able to reproduce the nil pointer dereference on any of my cluster but the error reported corresponded with the line that is changed in this PR.

OpenFaaS Pro Report
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x11bea09]

goroutine 1 [running]:
main.readFunctions({0xc0004ea000?, 0x11, 0x0?})
/go/src/[github.com/openfaas/checker/main.go:891](http://github.com/openfaas/checker/main.go:891) +0xc49
main.main()
/go/src/[github.com/openfaas/checker/main.go:423](http://github.com/openfaas/checker/main.go:423) +0x21c5

Verified the config-checker runs successfully on my clusters.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

Approved