[Core] Bug Fix: Movement Visual Error

You can find details of this bug here:
https://www.getmangos.eu/issue.php?issueid=130#note609

Apparently, when moving and being stun, the player has the flag MOVEFLAG_REDIRECTED.

Therefore, it tries to get more info from the packet but this one is empty.

NOTE: This not an ideal fix since we are checking the datasize directly.
      But it will do until a better solution is found.