b2b_entitites: do not unref cancelled cell if not ours

If a CANCEL is caught by the b2b_entities but it is not part of a b2b
dialog, it should not unref the cell, otherwise after the script is
executed, it will be unreffed again, leading to a double unref.

Many thanks to Voxtronic for spotting this issue!

modules/b2b_entities/dlg.c

@@ -885,13 +885,14 @@ int b2b_prescript_f(struct sip_msg *msg, void *uparam)
 		*/
 		dlg = b2bl_search_iteratively(&callid, &from_tag, T_invite,
 			hash_index);
-		tmb.unref_cell( T_invite );
 		if(dlg == NULL)
 		{
 			B2BE_LOCK_RELEASE(server_htable, hash_index);
 			LM_DBG("No dialog found for cancel\n");
 			return SCB_RUN_ALL;
 		}
+		tmb.unref_cell( T_invite );
+		reset_cancelled_t();
 
 		ctx = b2b_get_context();
 		if (!ctx) {