Fix #22661: Multires/Sculpt Segfult

- Show error message in multires modifier if there is no MDISPS layer
- Sculpt on basis mesh if there is no the same layer

source/blender/editors/sculpt_paint/sculpt.c

@@ -112,8 +112,14 @@ void ED_sculpt_force_update(bContext *C)
    it's the last modifier on the stack and it is not on the first level */
 struct MultiresModifierData *sculpt_multires_active(Scene *scene, Object *ob)
 {
+	Mesh *me= (Mesh*)ob->data;
 	ModifierData *md, *nmd;
-	
+
+	if(!CustomData_get_layer(&me->fdata, CD_MDISPS)) {
+		/* multires can't work without displacement layer */
+		return NULL;
+	}
+
 	for(md= modifiers_getVirtualModifierList(ob); md; md= md->next) {
 		if(md->type == eModifierType_Multires) {
 			MultiresModifierData *mmd= (MultiresModifierData*)md;

source/blender/modifiers/intern/MOD_multires.c

@@ -38,6 +38,8 @@
 #include "BKE_paint.h"
 #include "BKE_particle.h"
 
+#include "DNA_mesh_types.h"
+
 static void initData(ModifierData *md)
 {
 	MultiresModifierData *mmd = (MultiresModifierData*)md;
@@ -66,6 +68,15 @@ static DerivedMesh *applyModifier(ModifierData *md, Object *ob, DerivedMesh *dm,
 	int sculpting= (ob->mode & OB_MODE_SCULPT) && ss;
 	MultiresModifierData *mmd = (MultiresModifierData*)md;
 	DerivedMesh *result;
+	Mesh *me= (Mesh*)ob->data;
+
+	if(mmd->totlvl) {
+		if(!CustomData_get_layer(&me->fdata, CD_MDISPS)) {
+			/* multires can't work without displacement layer */
+			modifier_setError(md, "Modifier needs mesh with displacement data.");
+			return dm;
+		}
+	}
 
 	result = multires_dm_create_from_derived(mmd, 0, dm, ob, useRenderParams, isFinalCalc);