Fixed -1 to implicitly unsigned char narrowing error.

Fixed -1 to implicitly unsigned char narrowing error.

C++11 does not allow narrowing a constant expression value to a type
that can't fully represent it. So when char is considered unsigned,
we can't store -1 in it. Explicitly using signed char fixes it.

Change-Id: I5c0e9fe0025659e06291655a12220ab589d5f5bd
Reviewed-on: https://swiftshader-review.googlesource.com/9630
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>