MIPv6: Mobility Option Mobile Node Link Layer Identifier Link-layer Identifier field is read beyond the option data

there's a single IPv6 packet with Mobile Node Link Layer Identifier mobility option.
The option's data length is 9 but and since the Reserved field takes 2 bytes, the Link Layer Identifier field should take 7 bytes.
However, the Link Layer Identifier field is read until the end of the packet, which is 519 bytes.

Bug:10578
Change-Id: I518de7f6b75e5b749cb029845fd4846954ca6d0e
Reviewed-on: https://code.wireshark.org/review/4818
Reviewed-by: Michael Mann <mmann78@netscape.net>