fix: return undefined when attempting to download nonexistent media

This could occur when the user has deleted the media and it can no longer be downloaded.

src/structures/Message.js

@@ -190,8 +190,14 @@ class Message extends Base {
             const msg = window.Store.Msg.get(msgId);
             
             if(msg.mediaData.mediaStage != 'RESOLVED') {
+                // try to resolve media
                 await msg.downloadMedia(true, 1);
             }
+            
+            if(msg.mediaData.mediaStage != 'RESOLVED') {
+                // media could not be downloaded
+                return undefined;
+            }
 
             const buffer = await window.WWebJS.downloadBuffer(msg.clientUrl);
             const decrypted = await window.Store.CryptoLib.decryptE2EMedia(msg.type, buffer, msg.mediaKey, msg.mimetype);