drm/xe/guc: Don't read data from G2H prior to length check

While highly unlikely, incoming G2H message might be too short
so we shouldn't read any data from it prior to checking a length.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241105173032.1947-4-michal.wajdeczko@intel.com