st/mesa: fix size miss match for some check

While we shrink some variable from "GLuint" to "ubyte", need to update the check from "x != ~0U" to "x != 0xff" too.

This fixes the crash for SPECviewperf 13 benchmark medical case.

Fixes: d947e3e2 "st/mesa: decrease the size of st_vertex_program" Signed-off-by: Qiang Yu yuq825@gmail.com