brw_fs: Avoid zero size vla

brw_fs: Avoid zero size vla

../src/intel/compiler/brw_fs.cpp:2247:46: runtime error: variable length array bound evaluates to non-positive value 0
    #0 0x7f78f5697678 in fs_visitor::assign_constant_locations() ../src/intel/compiler/brw_fs.cpp:2247
    #1 0x7f78f571d29e in fs_visitor::optimize() ../src/intel/compiler/brw_fs.cpp:7361
    #2 0x7f78f574eb84 in fs_visitor::run_fs(bool, bool) ../src/intel/compiler/brw_fs.cpp:8022
    #3 0x7f78f575641b in brw_compile_fs ../src/intel/compiler/brw_fs.cpp:8408
    #4 0x7f78f255c8e4 in brw_codegen_wm_prog ../src/mesa/drivers/dri/i965/brw_wm.c:123
    #5 0x7f78f2565571 in brw_fs_precompile ../src/mesa/drivers/dri/i965/brw_wm.c:608
    #6 0x7f78f24edd2c in brw_shader_precompile ../src/mesa/drivers/dri/i965/brw_link.cpp:56
    #7 0x7f78f24f3af8 in brw_link_shader ../src/mesa/drivers/dri/i965/brw_link.cpp:381
    #8 0x7f78f39a302a in _mesa_glsl_link_shader ../src/mesa/program/ir_to_mesa.cpp:3119
    #9 0x7f78f3a43826 in create_new_program ../src/mesa/main/ff_fragment_shader.cpp:1133
    #10 0x7f78f3a43d00 in _mesa_get_fixed_func_fragment_program ../src/mesa/main/ff_fragment_shader.cpp:1163
    #11 0x7f78f325ddcd in update_program ../src/mesa/main/state.c:134
    #12 0x7f78f325fe64 in _mesa_update_state_locked ../src/mesa/main/state.c:360
    #13 0x7f78f32600f1 in _mesa_update_state ../src/mesa/main/state.c:394
    #14 0x7f78f2b3e587 in clear ../src/mesa/main/clear.c:169
    #15 0x7f78f2b3e587 in _mesa_Clear ../src/mesa/main/clear.c:242

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3825>