MIPv6: Mobility Option Binding Authorization Data for FMIPv6 Authenticator field is read beyond the option data

There's a single IPv6 packet with Binding Authorization Data for FMIPv6 mobility option.
The option's data length is 14 but and since the SPI takes 4 bytes, the Authenticator field should take 10 bytes.
However, the Authenticator field is read until the end of the packet, which is 178 bytes.

Found by boaz
Bug: 10577
Change-Id: Ia79c5131d8477c1a7c111c47c2a94e990bbeae4d
Reviewed-on: https://code.wireshark.org/review/4817
Reviewed-by: Michael Mann <mmann78@netscape.net>