intel: decoder: handle 0 sized structs

Gen7.5 has a BLEND_STATE of size 0 which includes a variable length
group. We did not deal with that very well, leading to an endless
loop.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107544


Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 440a988b)