-Bugfix #3207: ALT-C on a mesh with modifiers would cause memory corruption.

We make a copy of the mesh and perform operations on the new object using
   mesh_create_derived_no_deform() for the new object instead of the
   original.

   Would be really nice if Daniel would look this over to see if it's really
   fixed, or if I've just moved the problem somewhere else. :-)  I'm leaving
   the bug report open until the fixed is verified.

source/blender/src/editobject.c

@@ -2248,7 +2248,6 @@ void convertmenu(void)
 			
 			if(ob->flag & OB_DONE);
 			else if(ob->type==OB_MESH) {
-				Mesh *oldme= ob->data;
 				DispListMesh *dlm;
 				DerivedMesh *dm;
 
@@ -2265,22 +2264,17 @@ void convertmenu(void)
 				BLI_addhead(&G.scene->base, basen);	/* addhead: otherwise eternal loop */
 				basen->object= ob1;
 				basen->flag &= ~SELECT;
-					
+
+				/* decrement original mesh's usage count  */
 				me= ob1->data;
 				me->id.us--;
-					
-				ob1->data= add_mesh();
+
+				/* make a new copy of the mesh */
+				ob1->data= copy_mesh(me);
 				G.totmesh++;
-				ob1->type= OB_MESH;
 
-				me= ob1->data;
-				me->totcol= oldme->totcol;
-				if(ob1->totcol) {
-					me->mat= MEM_dupallocN(oldme->mat);
-					for(a=0; a<ob1->totcol; a++) id_us_plus((ID *)me->mat[a]);
-				}
-
-				dm= mesh_create_derived_no_deform(ob, NULL);
+				/* make new mesh data from the original copy */
+				dm= mesh_create_derived_no_deform(ob1, NULL);
 				dlm= dm->convertToDispListMesh(dm, 0);
 				displistmesh_to_mesh(dlm, ob1->data);
 				dm->release(dm);
@@ -4458,7 +4452,7 @@ void selectlinks(int nr)
 		mat= give_current_material(ob, ob->actcol);
 		if(mat==0) return;
 		if(nr==4) {
-			if(mat->mtex[ mat->texact ]) tex= mat->mtex[ mat->texact ]->tex;
+			if(mat->mtex[ (int)mat->texact ]) tex= mat->mtex[ (int)mat->texact ]->tex;
 			if(tex==0) return;
 		}
 	}