store: correctly remove incomplete layers on load.

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>

layers.go

@@ -827,22 +827,31 @@ func (r *layerStore) load(lockedForWriting bool) (bool, error) {
 		// user of this storage area marked for deletion but didn't manage to
 		// actually delete.
 		var incompleteDeletionErrors error // = nil
+		var layersToDelete []*Layer
 		for _, layer := range r.layers {
 			if layer.Flags == nil {
 				layer.Flags = make(map[string]interface{})
 			}
 			if layerHasIncompleteFlag(layer) {
-				logrus.Warnf("Found incomplete layer %#v, deleting it", layer.ID)
-				err := r.deleteInternal(layer.ID)
-				if err != nil {
-					// Don't return the error immediately, because deleteInternal does not saveLayers();
-					// Even if deleting one incomplete layer fails, call saveLayers() so that other possible successfully
-					// deleted incomplete layers have their metadata correctly removed.
-					incompleteDeletionErrors = multierror.Append(incompleteDeletionErrors,
-						fmt.Errorf("deleting layer %#v: %w", layer.ID, err))
-				}
-				modifiedLocations |= layerLocation(layer)
+				// Important: Do not call r.deleteInternal() here. It modifies r.layers
+				// which causes unexpected side effects while iterating over r.layers here.
+				// The range loop has no idea that the underlying elements where shifted
+				// around.
+				layersToDelete = append(layersToDelete, layer)
+			}
+		}
+		// Now actually delete the layers
+		for _, layer := range layersToDelete {
+			logrus.Warnf("Found incomplete layer %q, deleting it", layer.ID)
+			err := r.deleteInternal(layer.ID)
+			if err != nil {
+				// Don't return the error immediately, because deleteInternal does not saveLayers();
+				// Even if deleting one incomplete layer fails, call saveLayers() so that other possible successfully
+				// deleted incomplete layers have their metadata correctly removed.
+				incompleteDeletionErrors = multierror.Append(incompleteDeletionErrors,
+					fmt.Errorf("deleting layer %#v: %w", layer.ID, err))
 			}
+			modifiedLocations |= layerLocation(layer)
 		}
 		if err := r.saveLayers(modifiedLocations); err != nil {
 			return false, err