test_vendor: L2cap Assemble additional error check

Assemble now properly checks the proposed length of a segmented SDU that
is given in the first packet of any segmented SDU in accordance with
Bluetooth Core Specification Version 4.2 Volume 3 Part A 3.3.2

Test: Passed in test/l2cap_test.cc
Change-Id: Idbbd1c014bac8928affab581118f73a7d9126824