WCCP: fix parsing of extended assignment data element

Bug: 10641
Change-Id: I51322757c5aa7877fa8ff7915330ff24e7a8cbe2
Reviewed-on: https://code.wireshark.org/review/5221
Reviewed-by: Michael Mann <mmann78@netscape.net>
(cherry picked from commit 0f758fa2fcfac59601c13e5994ea5f848347741d)
Change-Id: I51322757c5aa7877fa8ff7915330ff24e7a8cbe2
Reviewed-on: https://code.wireshark.org/review/5223
Reviewed-by: Michael Mann <mmann78@netscape.net>