util/nvmutil: safer SSIZE_MAX define

the current one assumes two's compliment and no
padding bits. i assert two's compliment earlier
in code, but it doesn't guarantee:

sizeof(ssize_t) == sizeof(size_t)

it's theoretically possible that size_t=64
and ssize_t=32, and then the macro would break.

this new version uses SIZE_MAX instead, without
subtraction, but halves it using a bit shift.

this may still break, but it should work nicely.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -289,7 +289,7 @@ static const char *fname;
 static const char *argv0;
 
 #ifndef SSIZE_MAX
-#define SSIZE_MAX ((ssize_t)((size_t)-1 >> 1))
+#define SSIZE_MAX ((ssize_t)(SIZE_MAX >> 1))
 #endif
 
 /*