Fix texture sampling buffer overrun.

Sampling byte4 data currently reads 8 bytes for unpacking purposes.
Allocate 4 more bytes to prevent reading outside the image, even
though it's unused data.

Bug 21935792

Change-Id: I162fb3f3575131cedb008f82ef5170e773719e41
Reviewed-on: https://swiftshader-review.googlesource.com/3572
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>