i965: Fix out-of-bounds accesses into pull_constant_loc array
i965: Fix out-of-bounds accesses into pull_constant_loc array

The piglit test glsl-fs-uniform-array-loop-unroll.shader_test was designed
to do an out of bounds access into an uniform array to make sure that we
handle that situation gracefully inside the driver, however, as Ken describes
in bug 79202, Valgrind reports that this is leading to an out-of-bounds access
in fs_visitor::demote_pull_constants().

Before accessing the pull_constant_loc array we should make sure that
the uniform we are trying to access is valid.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79202
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler