Fix 500 when joining matrix-dev

matrix-dev has an event (`$/6ANj/9QWQyd71N6DpRQPf+SDUu11+HVMeKSpMzBCwM:zemos.net`)
which has no `hashes` member.

Check for missing `hashes` element in events.

synapse/crypto/event_signing.py

@@ -32,15 +32,22 @@ def check_event_content_hash(event, hash_algorithm=hashlib.sha256):
     """Check whether the hash for this PDU matches the contents"""
     name, expected_hash = compute_content_hash(event, hash_algorithm)
     logger.debug("Expecting hash: %s", encode_base64(expected_hash))
-    if name not in event.hashes:
+
+    # some malformed events lack a 'hashes'. Protect against it being missing
+    # or a weird type by basically treating it the same as an unhashed event.
+    hashes = event.get("hashes")
+    if not isinstance(hashes, dict):
+        raise SynapseError(400, "Malformed 'hashes'", Codes.UNAUTHORIZED)
+
+    if name not in hashes:
         raise SynapseError(
             400,
             "Algorithm %s not in hashes %s" % (
-                name, list(event.hashes),
+                name, list(hashes),
             ),
             Codes.UNAUTHORIZED,
         )
-    message_hash_base64 = event.hashes[name]
+    message_hash_base64 = hashes[name]
     try:
         message_hash_bytes = decode_base64(message_hash_base64)
     except Exception: