MIPv6: Mobility Option IPv6 Address/Prefix marks too many bytes for the address/prefix field

One of the mobility options is the IPv6 Address/Prefix.
The prefix length of this option is 52.
Note that according to RFC 5568, the prefix length is in bits and not in bytes.
The IPv6 Address/Prefix field of this option marks 52 bytes.

In addition, I'm not sure that the IPv6 Address/Prefix field is a variable size field.
RFC 5568 section 6.4.2 has a diagram of this options that shows this field as if it always takes 128 bits, and the prefix length field just says the length of the prefix (not the length of the field).

Found by boaz

Bug:10576
Change-Id: I2a9a35cb7c537ec3b91c9a089f14fa31f41d8c80
Reviewed-on: https://code.wireshark.org/review/4816
Reviewed-by: Michael Mann <mmann78@netscape.net>