Blender Git Loki

Fix for a segfault in the new Displace modifier UV layer selection code:
If the "UV" texture coordinate option is selected while no UV layers exist,
the UV layer name in the modifier is blank. This is not a problem while no UV
layers exist, but if a UV layer is added the modifier code attempts to use
UV coordinates without handling the missing layer name correctly, leading to a
segfault. This only occurs when the modifier stack is recalculated before a
modifier UI redraw, as the UI redraw updates the layer name.

This fix handles a missing UV layer name by setting it to the active UV layer.