Fix T39152: Blender crash when duplicate mesh with hooks and laplacian deform.

We can't simply dupalloc cache_system (LaplacianSystem), it has quite a few allocated data we'd need to dupalloc as well,
not to mention (nl) context... Much safer to just set it to NULL in new copy imho!

This commit is to be backported to 2.70 release!

source/blender/modifiers/intern/MOD_laplaciandeform.c

@@ -808,7 +808,7 @@ static void copyData(ModifierData *md, ModifierData *target)
 	modifier_copyData_generic(md, target);
 
 	tlmd->vertexco = MEM_dupallocN(lmd->vertexco);
-	tlmd->cache_system = MEM_dupallocN(lmd->cache_system);
+	tlmd->cache_system = NULL;
 }
 
 static bool isDisabled(ModifierData *md, int UNUSED(useRenderParams))