b2b_entities: fix crashes after running the script request route

After releasing the lock and running the script request route, check
if the dialog has not been deleted in the meantime.

Credits to David Escartin from Sonoc for reporting.

(cherry picked from commit 65a8da4)

modules/b2b_entities/dlg.c

@@ -1092,6 +1092,19 @@ int b2b_prescript_f(struct sip_msg *msg, void *uparam)
 			}
 		}
 		B2BE_LOCK_GET(table, hash_index);
+
+		/* check if the dialog has not be deleted while not holding the lock */
+		for(aux_dlg = table[hash_index].first; aux_dlg; aux_dlg = aux_dlg->next)
+		{
+			if(aux_dlg == dlg)
+				break;
+		}
+		if(!aux_dlg)
+		{
+			LM_DBG("Record not found anymore\n");
+			B2BE_LOCK_RELEASE(table, hash_index);
+			return SCB_DROP_MSG;
+		}
 	}
 
 	if(method_value != METHOD_CANCEL)