From Stephen Donnelly: a better fix for the fuzz failure reported in
Previous patch solved the reported problem. This patch should cover input
validation for the VC Id and Link Rate fields properly.
Note link_rate==0 (unknown) is supported, as some hardware/firmware may not set
this field, but vc_size==0 is defined invalid.
From me: also initialize m_sdh_line_rate when the input is garbage.