glsl: fix possible memory leak in nir uniform linker

Use UniformDataSlots for the context of UniformDataDefaults rather
than UniformStorage as in some cause UniformStorage may be NULL.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3992>

src/compiler/glsl/gl_nir_link_uniforms.c

@@ -55,7 +55,7 @@ nir_setup_uniform_remap_tables(struct gl_context *ctx,
    prog->data->UniformDataSlots = data;
 
    prog->data->UniformDataDefaults =
-         rzalloc_array(prog->data->UniformStorage,
+         rzalloc_array(prog->data->UniformDataSlots,
                        union gl_constant_value, prog->data->NumUniformDataSlots);
 
    unsigned data_pos = 0;