glsl: fix leak in gl_nir_link_uniform_blocks

asan on llvmpipe with piglit tests/spec/arb_gl_spirv/execution/ssbo/array-indirect.shader_test reported.

==3288325==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 48 byte(s) in 1 object(s) allocated from:
    #0 0x7f5b2d6513cf in __interceptor_malloc (/lib64/libasan.so.6+0xab3cf)
    #1 0x7f5b2a1ae810 in ralloc_size ../src/util/ralloc.c:133
    #2 0x7f5b2a1ae7e1 in ralloc_context ../src/util/ralloc.c:120
    #3 0x7f5b2b210177 in gl_nir_link_uniform_blocks ../src/compiler/glsl/gl_nir_link_uniform_blocks.c:585
    #4 0x7f5b2af7f52d in gl_nir_link_spirv ../src/compiler/glsl/gl_nir_linker.c:614
    #5 0x7f5b2a3b76fa in st_link_nir ../src/mesa/state_tracker/st_glsl_to_nir.cpp:765
    #6 0x7f5b2a3ace7b in st_link_shader ../src/mesa/state_tracker/st_glsl_to_ir.cpp:65
    #7 0x7f5b2a471165 in _mesa_glsl_link_shader ../src/mesa/program/ir_to_mesa.cpp:3122
    #8 0x7f5b2a97a6d8 in link_program ../src/mesa/main/shaderapi.c:1311
    #9 0x7f5b2a97a6d8 in link_program_error ../src/mesa/main/shaderapi.c:1419
    #10 0x7f5b2a97df45 in _mesa_LinkProgram ../src/mesa/main/shaderapi.c:1911
    #11 0x7f5b299b59e5 in stub_glLinkProgram /mnt/devel/gl/piglit/tests/util/piglit-dispatch-gen.c:33956
    #12 0x40a71a in link_and_use_shaders /mnt/devel/gl/piglit/tests/shaders/shader_runner.c:1604
    #13 0x415722 in init_test /mnt/devel/gl/piglit/tests/shaders/shader_runner.c:5225
    #14 0x4164ce in piglit_init /mnt/devel/gl/piglit/tests/shaders/shader_runner.c:5597
    #15 0x7f5b29a214e9 in run_test /mnt/devel/gl/piglit/tests/util/piglit-framework-gl/piglit_winsys_framework.c:73
    #16 0x7f5b29a103fe in piglit_gl_test_run /mnt/devel/gl/piglit/tests/util/piglit-framework-gl.c:229
    #17 0x407847 in main /mnt/devel/gl/piglit/tests/shaders/shader_runner.c:72
    #18 0x7f5b2928f1e1 in __libc_start_main (/lib64/libc.so.6+0x281e1)

SUMMARY: AddressSanitizer: 48 byte(s) leaked in 1 allocation(s).

Fixes: 57239192 ("nir/linker: add gl_nir_link_uniform_blocks.c")

Closes: #4274 (closed)