Fatal error on editing/displaying map when one of its layers does no longer exists

Problem/Motivation

When a map contains manually added layers, one of those layers can be deleted. Loading the map to be displayed or edited results in an fatal error.

Steps to reproduce

• Add some layers using the layers UI.
• Add a map using one or more of these layers.
• Remove one of the layers used in the map.
• The map can no longer be edited nor displayed.

Proposed resolution

Add an extra check, in the leaflet_layers_leaflet_map_info() hook, if the layer can be loaded.