L2CAP ERTM: Drop invalid packet on reassembly

Store the expected SDU size on a START I-Frame.

If the reassembled SDU size is not equal to expected SDU size, drop the
SDU. Also don't allow receiving UNSEGMENTED PDU in the middle of
reassembly.

In next iteration, close the channel on receiving some invalid frame.

Bug: 144955385
Test: bluetooth_test_gd
Change-Id: Id36aa242802b6991c59c9f9f45e8fac064b26675