Fix mob deserialization errors in the client (#4743)

Fix mob deserialization errors in the client (#4743)

The problem was seen while using the mobf mod package.

The problem happens when the server serializes entity attachments.
Sometimes, such attachments no longer exist. The serialization code
skips those. However, the total number of attachments was serialized
earlier.  Therefore the client expects more than it gets, and logs a
serialization error.