Add more boundary checking to sse3/4 parsing

ssse3 uses tables with only two entries per op, but it is indexed
with b1 which can contain variables upto 3. This happens when ssse3
or sse4 are used with REP* prefixes.

Add boundary checking for this case.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
