compiler: glsl: silence valgrind warning on write cache

compiler: glsl: silence valgrind warning on write cache

I don't think it actually fixes anything, but that's nice not to have valgrind warnings.
It manifests itself when running the piglit test : glsl-fs-raytrace-bug27060

==2058== Uninitialised byte(s) found during client check request
==2058==    at 0xC5BB040: blob_write_bytes (blob.c:152)
==2058==    by 0xC595359: write_variable (nir_serialize.c:144)
==2058==    by 0xC59560C: write_var_list (nir_serialize.c:192)
==2058==    by 0xC5982E4: nir_serialize (nir_serialize.c:1124)
==2058==    by 0xC0B729D: brw_program_serialize_nir (brw_program.c:835)
==2058==    by 0xC0AB2D6: brw_link_shader (brw_link.cpp:358)
==2058==    by 0xC32FE3F: _mesa_glsl_link_shader (ir_to_mesa.cpp:3169)
==2058==    by 0xC36C7ED: create_new_program(gl_context*, state_key*) (ff_fragment_shader.cpp:1127)
==2058==    by 0xC36C8A6: _mesa_get_fixed_func_fragment_program (ff_fragment_shader.cpp:1157)
==2058==    by 0xC1B50AF: update_program (state.c:134)
==2058==    by 0xC1B56DF: _mesa_update_state_locked (state.c:352)
==2058==    by 0xC1B579A: _mesa_update_state (state.c:386)
==2058==  Address 0xf1eab8a is 58 bytes inside a block of size 96 alloc'd
==2058==    at 0x4C2CB8F: malloc (vg_replace_malloc.c:299)
==2058==    by 0xC0FD306: ralloc_size (ralloc.c:121)
==2058==    by 0xC0FD5B1: ralloc_array_size (ralloc.c:208)
==2058==    by 0xC452B3B: (anonymous namespace)::nir_visitor::visit(ir_variable*) (glsl_to_nir.cpp:448)
==2058==    by 0xC45CE8B: ir_variable::accept(ir_visitor*) (ir.h:428)
==2058==    by 0xC46D0B5: visit_exec_list(exec_list*, ir_visitor*) (ir.cpp:1898)
==2058==    by 0xC451D2F: glsl_to_nir (glsl_to_nir.cpp:162)
==2058==    by 0xC0B5223: brw_create_nir (brw_program.c:79)
==2058==    by 0xC0AAB67: brw_link_shader (brw_link.cpp:257)
==2058==    by 0xC32FE3F: _mesa_glsl_link_shader (ir_to_mesa.cpp:3169)
==2058==    by 0xC36C7ED: create_new_program(gl_context*, state_key*) (ff_fragment_shader.cpp:1127)
==2058==    by 0xC36C8A6: _mesa_get_fixed_func_fragment_program (ff_fragment_shader.cpp:1157)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>