st/glsl_to_tgsi: avoid iterating past the head of the instruction list

st/glsl_to_tgsi: avoid iterating past the head of the instruction list

exec_node::get_prev() does not guard against going past the beginning
of the list, so we need to add explicit checks here.

Found by ASAN in piglit arb_shader_storage_buffer_object-rendering.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Marek Olšák <marek.olsak@amd.com>