Backend/moderation: use .get for from_visibility lookup

Avoids KeyError on unknown enum ints; the None-in-set check already covers it.

Author: aapeliv

app/backend/src/couchers/servicers/moderation.py

@@ -542,7 +542,7 @@ def SetUserContentVisibility(
         if new_visibility is None:
             context.abort_with_error_code(grpc.StatusCode.INVALID_ARGUMENT, "visibility_must_be_specified")
 
-        from_visibilities = {moderationvisibility2sql[v] for v in request.from_visibility}
+        from_visibilities = {moderationvisibility2sql.get(v) for v in request.from_visibility}
         if None in from_visibilities:
             context.abort_with_error_code(grpc.StatusCode.INVALID_ARGUMENT, "visibility_must_be_specified")