main: Fix memleaks in mesa_use_program

Add freeing of SubroutineIndexes to the _mesa_free_shader_state.

Fixes: 4566aaaa5b ("mesa/subroutines: start adding per-context
subroutine index support (v1.1)")
Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/main/shaderapi.c

@@ -164,6 +164,8 @@ _mesa_free_shader_state(struct gl_context *ctx)
       _mesa_reference_shader_program(ctx,
                                      &ctx->Shader.ReferencedPrograms[i],
                                      NULL);
+      free(ctx->SubroutineIndex[i].IndexPtr);
+      ctx->SubroutineIndex[i].IndexPtr = NULL;
    }
    _mesa_reference_shader_program(ctx, &ctx->Shader.ActiveProgram, NULL);