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.

source/blender/blenkernel/intern/modifier.c

@@ -2315,6 +2315,11 @@ static void get_texture_coords(DisplaceModifierData *dmd, Object *ob,
 					tf = dm->faceData.layers[itf].data;
 					strcpy(dmd->uvlayer_name, dm->faceData.layers[itf].name);
 				}
+			} else {
+				/* no uv layer specified, use the active one */
+				itf = CustomData_get_active_layer_index(&dm->faceData, CD_MTFACE);
+				tf = dm->faceData.layers[itf].data;
+				strcpy(dmd->uvlayer_name, dm->faceData.layers[itf].name);
 			}
 			
 			/* verts are given the UV from the first face that uses them */