Ensure a sid parameter is passed to bind_threepid (#5995)

`sid` is required to be part of `three_pid_creds`. We were 500'ing if it wasn't provided instead of returning `M_MISSING_PARAM`.

Author: anoadragon453

changelog.d/5995.bugfix

@@ -0,0 +1 @@
+Return a M_MISSING_PARAM if `sid` is not provided to `/account/3pid`.

synapse/handlers/identity.py

@@ -151,12 +151,18 @@ def bind_threepid(self, creds, mxid, use_v2=True):
             creds
         )
 
+        sid = creds.get("sid")
+        if not sid:
+            raise SynapseError(
+                400, "No sid in three_pid_creds", errcode=Codes.MISSING_PARAM
+            )
+
         # If an id_access_token is not supplied, force usage of v1
         if id_access_token is None:
             use_v2 = False
 
         # Decide which API endpoint URLs to use
-        bind_data = {"sid": creds["sid"], "client_secret": client_secret, "mxid": mxid}
+        bind_data = {"sid": sid, "client_secret": client_secret, "mxid": mxid}
         if use_v2:
             bind_url = "https://%s/_matrix/identity/v2/3pid/bind" % (id_server,)
             bind_data["id_access_token"] = id_access_token