Fix AARP AppleTalk address parsing

AppleTalk addresses are 3 bytes long and stored in AARP packets as 4 bytes.
The high byte should be 0, followed by 2-byte network number, followed by
1-byte node number.

The previous code was assuming that the high two bytes were the network number,
followed by the 1-byte node number, followed by 0.

Change-Id: I467ec6edac353796db0b96fbac65658d5c5491d3
Reviewed-on: https://code.wireshark.org/review/5968
Reviewed-by: Guy Harris <guy@alum.mit.edu>