mesa/st: fix possible crash related to arb invalid memory access

mesa/st: fix possible crash related to arb invalid memory access

This invalid memory access is a consequence of wrong assumptions,
for instance:
"prog->sh.data is NULL if it's ARB_fragment_program"

This issue is triggered with piglit/fp-formats -auto -fbo:
==9747==ERROR: AddressSanitizer: heap-use-after-free on address 0x007f7c812d90 at pc 0x007f833c09f8 bp 0x007fd7eca750 sp 0x007fd7eca768
READ of size 4 at 0x007f7c812d90 thread T0
    #0 0x7f833c09f4 in st_get_sampler_views ../src/mesa/state_tracker/st_atom_texture.c:109
    #1 0x7f833c0b48 in update_textures ../src/mesa/state_tracker/st_atom_texture.c:266
    #2 0x7f82b2d120 in st_validate_state ../src/mesa/state_tracker/st_util.h:128
    #3 0x7f82b2d120 in prepare_draw ../src/mesa/state_tracker/st_draw.c:88
    #4 0x7f82b2de64 in st_draw_gallium ../src/mesa/state_tracker/st_draw.c:141
    #5 0x7f83105940 in _mesa_draw_arrays ../src/mesa/main/draw.c:1202
    #6 0x7f8d5fa5cc in piglit_draw_rect_from_arrays piglit/tests/util/piglit-util-gl.c:711
    #7 0x7f8d5fac34 in piglit_draw_rect_custom piglit/tests/util/piglit-util-gl.c:833
    #8 0x4019e0 in piglit_display piglit/tests/shaders/fp-formats.c:67
    #9 0x7f8d643fc4 in run_test piglit/tests/util/piglit-framework-gl/piglit_fbo_framework.c:52
    #10 0x401624 in main piglit/tests/shaders/fp-formats.c:39

Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21175>