Fix some potential out_of_range json errors

lib/structs/events/canonical_alias.cpp

@@ -13,7 +13,7 @@ namespace state {
 void
 from_json(const json &obj, CanonicalAlias &canonical_alias)
 {
-        if (!obj.at("alias").is_null())
+        if (obj.find("alias") != obj.end() && !obj.at("alias").is_null())
                 canonical_alias.alias = obj.at("alias").get<std::string>();
 }
 

lib/structs/events/name.cpp

@@ -12,7 +12,7 @@ namespace state {
 void
 from_json(const json &obj, Name &event)
 {
-        if (!obj.at("name").is_null())
+        if (obj.find("name") != obj.end() && !obj.at("name").is_null())
                 event.name = obj.at("name").get<std::string>();
 }
 

lib/structs/events/topic.cpp

@@ -11,7 +11,7 @@ namespace state {
 void
 from_json(const json &obj, Topic &event)
 {
-        if (!obj.at("topic").is_null())
+        if (obj.find("topic") != obj.end() && !obj.at("topic").is_null())
                 event.topic = obj.at("topic").get<std::string>();
 }
 

lib/structs/responses/notifications.cpp

@@ -14,7 +14,7 @@ from_json(const json &obj, Notification &res)
         res.room_id = obj.at("room_id");
         res.ts      = obj.at("ts");
 
-        if (!obj.at("profile_tag").is_null())
+        if (obj.find("profile_tag") != obj.end() && !obj.at("profile_tag").is_null())
                 res.profile_tag = obj.at("profile_tag");
 
         // HACK to work around the fact that there isn't