pan/midgard: Remove undefined behavior

pan/midgard: Remove undefined behavior

As found by UBSAN, it should be harmless but it's good to remove any UB
so the tool's output is useful.

../src/panfrost/midgard/midgard_schedule.c:1094:9: runtime error: index -1 out of bounds for type 'midgard_instruction *[6]'"}
    #0 0xad047872 in schedule_block ../src/panfrost/midgard/midgard_schedule.c:1094"}
    #1 0xad04d41a in schedule_program ../src/panfrost/midgard/midgard_schedule.c:1116"}
    #2 0xad031f98 in midgard_compile_shader_nir ../src/panfrost/midgard/midgard_compile.c:2588"}
    #3 0xacf9874e in panfrost_shader_compile ../src/gallium/drivers/panfrost/pan_assemble.c:68"}
    #4 0xacf6b268 in panfrost_bind_shader_state ../src/gallium/drivers/panfrost/pan_context.c:1960"}
    #5 0xaae2596e in st_update_fp ../src/mesa/state_tracker/st_atom_shader.c:168"}
    #6 0xaae12316 in st_validate_state ../src/mesa/state_tracker/st_atom.c:261"}
    #7 0xaadc58c2 in prepare_draw ../src/mesa/state_tracker/st_draw.c:132"}
    #8 0xaadc58c2 in st_draw_vbo ../src/mesa/state_tracker/st_draw.c:184"}
    #9 0xabc4f924 in _mesa_validated_drawrangeelements ../src/mesa/main/draw.c:816"}
    #10 0xabc50240 in _mesa_DrawElements ../src/mesa/main/draw.c:970"}
    #11 0x73ebd2 in glu::CallLogWrapper::glDrawElements(unsigned int, int, unsigned int, void const*) (/deqp/modules/gles2/deqp-gles2+0x2d4bd2)"}
    #12 0x6d86b2 in deqp::gls::FragOpInteractionCase::iterate() (/deqp/modules/gles2/deqp-gles2+0x26e6b2)"}
    #13 0x494d16 in deqp::gles2::TestCaseWrapper::iterate(tcu::TestCase*) (/deqp/modules/gles2/deqp-gles2+0x2ad16)"}
    #14 0x7f9cf2 in tcu::TestSessionExecutor::iterateTestCase(tcu::TestCase*) (/deqp/modules/gles2/deqp-gles2+0x38fcf2)"}
    #15 0x7fa5f0 in tcu::TestSessionExecutor::iterate() (/deqp/modules/gles2/deqp-gles2+0x3905f0)"}
    #16 0x7e1aac in tcu::App::iterate() (/deqp/modules/gles2/deqp-gles2+0x377aac)"}
    #17 0x492d4c in main (/deqp/modules/gles2/deqp-gles2+0x28d4c)"}
    #18 0xb64b9aa8 in __libc_start_main (/lib/arm-linux-gnueabihf/libc.so.6+0x1aaa8)"}

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>