In go one should never modify a slice while also iterating over it at
the same time. This causes weird side effects as the underlying array
elements are shifted around without the range loop index knowing.
So if you delete a element the loop will then actually skip the next one
and theoretically access out of bounds on the last element which does
not panic but rather return the default zero type, nil here which then
causes the panic on layer.Flags == nil.

Here is a simple example to show the behavior:
func main() {
	slice := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
	for _, num := range slice {
		if num == 5 {
			slice = slices.DeleteFunc(slice, func(n int) bool {
				return n == 5
			})
		}
		fmt.Println(num)
	}
}

The loop will not print 6, but then as last number it prints 0 (the
default zero type for an int).

Fixes containers#2184

Signed-off-by: Paul Holzinger <pholzing@redhat.com>
(cherry picked from commit 99b0d2d)