st/glsl_to_tgsi: fix potential crash when allocating temporaries

st/glsl_to_tgsi: fix potential crash when allocating temporaries

When index - t->temps_size is greater than 4096, allocating space for
temporaries on demand will miserably crash. This can happen when a game
uses a lot of temporaries like the recent released Tomb raider.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>