tail should be equal to the last valid index, so
decrease it by one.

This error causes in "a gap" in some cases (as well as
some possible out-of-bound write), finally resulting in
ucode assertion.
Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>