st/mesa: allow negative index for PROGRAM_STATE_VAR

The piglit vp-address-01 test uses negative address-relative
offsets.  In this test we're indexing into PROGRAM_STATE_VAR
which, in turn, contains references to ENV vars.

We previously fixed this issue for PROGRAM_CONSTANT.

piglit/vp-address-01 (the version from Feb 5) passes now.

(cherry picked from commit 64be837b0b171c44f47a3c0b83f566d292ffff50)
1 file changed