Multires: Fix ownership problem causing crashes

Fixes crash when leaving multires mode.

Fixes T61836: Saving while in multires makes blender crash

source/blender/modifiers/intern/MOD_multires.c

@@ -168,6 +168,10 @@ static Mesh *applyModifier(ModifierData *md,
 		/* NOTE: CCG takes ownership over Subdiv. */
 		result = multires_as_ccg(mmd, ctx, mesh, subdiv);
 		result->runtime.subdiv_ccg_tot_level = mmd->totlvl;
+		/* NOTE: CCG becomes an owner of Subdiv descriptor, so can not share
+		 * this pointer. Not sure if it's needed, but might have a second look
+		 * on the ownership model here. */
+		mmd->subdiv = NULL;
 		// BKE_subdiv_stats_print(&subdiv->stats);
 	}
 	else {