Crash on room join

```
DEBUG 16:14:32:273 [ProfileSync] Checking for profile updates for @xmpp_user=40xmpp.domain.element.dev:subdomain.domain.element.d
INFO 16:14:32:273 [ProfileSync] Fetching user info for user@xmpp.domain.element.dev
INFO 16:14:32:273 [XmppJsInstance] Fetching vCard for user@xmpp.domain.element.dev
DEBUG 16:14:32:275 [XmppJsInstance] Stanza: {
name: 'iq',
attrs: {
type: 'result',
id: '27daecdc-1cd6-49a4-9963-1b2da5e9c826',
from: 'user@xmpp.domain.element.dev',
to: 'matrix.xmpp.domain.element.dev'
},
children: [ { name: 'vCard', attrs: [Object], children: [Array] } ]
}
INFO 16:14:32:275 [XmppJsInstance] Got iq from=user@xmpp.domain.element.dev to=matrix.xmpp.domain.element.dev isOurs=true
DEBUG 16:14:32:275 [ProfileSync] getUserInfo got: {
Nickname: 'user',
Avatar: 'image/jpeg|',
eventName: '-unused-',
who: 'user@xmpp.domain.element.dev',
account: { protocol_id: '', username: '' }
}
DEBUG 16:14:32:276 [ProfileSync] Got an avatar, setting
ERROR 16:14:32:276 [ProfileSync] Failed to update avatar_url for user: Error: Avatar uri was malformed
at XmppJsGateway.getAvatarBuffer (/app/src/xmppjs/XJSGateway.ts:511:19)
at ProfileSync.updateProfile (/app/src/ProfileSync.ts:84:52)
at runMicrotasks ()
at processTicksAndRejections (node:internal/process/task_queues:96:5)
at GatewayHandler.handleRoomJoin (/app/src/GatewayHandler.ts:193:17)
INFO 16:14:32:278 [GatewayHandler] Attempting to join #unencryptedpublic:subdomain.domain.element.dev
DEBUG 16:14:32:278 [MatrixHttpClient] (REQ-34) GET https://matrix.subdomain.domain.element.dev/_matrix/client/v3/directory/room/%23unen
DEBUG 16:14:32:283 [PgDatstore] Getting room !dOkmYxmKXexmUqNSGW:subdomain.domain.element.dev
DEBUG 16:14:32:284 [PgDatstore] Found room {"gateway":true,"room_name":"#unencryptedpublic#subdomain.domain.element.dev@matrix.xmpp.em
DEBUG 16:14:32:285 [XmppJsGateway] Handling remote join for PHByZXNlbmNlIHRvPSIjdW5lbmNyeXB0ZWRwdWJsaWMjcmhlbC5lbXMtc3VwcG9ydC5lbGVtZW5
INFO 16:14:32:285 [XmppJsGateway] Updating membership for #unencryptedpublic#subdomain.domain.element.dev@matrix.xmpp.domain.eleme
ERROR 16:14:32:285 [XmppJsGateway] Conflicting nickname, not joining
WARN 16:14:32:286 [GatewayHandler] Failed to join room: Conflicting nickname, not joining
DEBUG 16:14:32:286 [XmppJsGateway] Handling remote join for PHByZXNlbmNlIHRvPSIjdW5lbmNyeXB0ZWRwdWJsaWMjcmhlbC5lbXMtc3VwcG9ydC5lbGVtZW5
ERROR 16:14:32:286 [XmppJsGateway] Could not find stanza in cache for remoteJoin. Cannot handle
DEBUG 16:14:32:286 [MatrixHttpClient] (REQ-35) POST https://matrix.subdomain.domain.element.dev/_matrix/client/v3/rooms/!dOkmYxmKXexmUq
Error: Stanza for join not in cache, cannot handle
at XmppJsGateway.onRemoteJoin (/app/src/xmppjs/XJSGateway.ts:284:19)
at GatewayHandler.handleRoomJoin (/app/src/GatewayHandler.ts:231:39)
at runMicrotasks ()
at processTicksAndRejections (node:internal/process/task_queues:96:5)
Stream closed EOF for element-onprem/instance-bifrost-0 (bifrost)
```

Start by tracing GatewayHandler.handleRoomJoin in src/GatewayHandler.ts, then inspect XmppJsGateway.onRemoteJoin and getAvatarBuffer in src/xmppjs/XJSGateway.ts. Use the supplied join logs to follow the conflicting nickname and missing stanza-cache errors; done means the room join no longer crashes or leaves the join unhandled.

Written by the indexing model from the issue text.